Facile thermal synthesis of g–C3N4/ZnO nanocomposite with antibacterial properties for photodegradation of Methylene blue

Semiconductors as photocatalysts are ideal materials for wastewater remediation. A nanocomposite of g–C3N4 and ZnO was produced using a two-step in situ synthesis technique to achieve a better photocatalyst. The samples were assessed via UV–vis diffuse reflection spectroscopy, transmission electron microscopy, photoluminescence spectroscopy, Fourier transform infrared analysis, and x-ray diffraction. The photodegradation of methylene blue as an organic dye model was assessed to assess the photocatalytic characteristics of the fabricated samples. The antibacterial characteristics of synthesized samples were also investigated. The findings revealed that the photodegradation efficiency of the binary g–C3N4/ZnO systems was better than that of pure g–C3N4. Under irradiation, the photodegradation yield of g–C3N4/ZnO with a 15 wt.% of ZnO was up to 3.5 times better than that of pristine g–C3N4. The feature of enhanced separation of photoinduced holes and electrons resulting from heterojunction creation among g–C3N4 and ZnO surfaces might be attributed to this photocatalytic activity enhancement. The synthesized binary nanocomposites showed suitable antibacterial properties against Staphylococcus aureus and Escherichia coli bacteria.


Introduction
Consumption of lots of fossil fuels and production factory waste by populations have resulted in fast reduction of resources and alarming levels of environmental pollution in the modern world [1][2][3]. Water pollution is currently being recognized as a major problem for humans, as it is regularly affected by a variety of harmful chemicals from industries such as textiles, cosmetics, food, and paint [4][5][6]. There are various ways available for dealing with contaminated water in this respect, but visible-light-activated photocatalysis procedures are regarded to be a successful strategy that may have captivated the world due to its limitless solar energy [7][8][9]. Photocatalytic characteristics of semiconductors are highly dependent on their intrinsic physicochemical natures, comprising their band-gap position, surface properties, particle size, etc [10][11][12][13].
The most stable form of C 3 N 4 , polymeric g-C 3 N 4 or g-C 3 N 4 (g stands for graphitic), with a band gap of around 2.7 eV [14], has potential applications for the energy industry [15,16], hydrogen generation [17][18][19][20], gas sensors [21,22], and solar cells [23][24][25]. The g-C 3 N 4 has recently been studied as a visible-light-activated semiconductor for splitting H 2 O into O 2 and H 2 molecules under visible light [26][27][28][29][30][31][32]. It also works as a catalyst for the removal and photodegradation of organic-based pollutants in H 2 O [33,34] or in the air [35]. The g-C 3 N 4 semiconductor has desirable and useful properties for different applications because of its great chemical and thermal stability. Nevertheless, fast recombination of photoproduced electron-hole (e-h) pairs is a problem in this material. This phenomenon reduces the yield of catalytic efficiency of bare g-C 3 N 4 [36,37].
Synthesis of nanosheet with mesoporous structures [38], alteration via reversible protonation [39], adding different elements [40,41], combining with conductive nanomaterials like graphene [42] and noble metal [43], and adding other semiconductor materials [44,45] have all been tried to ameliorate the catalytic efficiency of g-C 3 N 4 under visible light. For instance, Zhang et al [46] produced g-C 3 N 4 /LaFeO 3 /Ag nanocomposite with a Z-scheme heterojunction. They showed that the prepared nanosystem exhibits high photocatalytic activity for the photodegradation of methylene blue and tetracycline hydrochloride. Tian et al [47] synthesized twodimensional g-C 3 N 4 /MoS 2 /graphene tertiary composite via an in situ adsorption method and showed the prepared samples have high photodegradation activities under visible light for the removal of Rhodamine B.
One of the most efficient ways for separating photogenerated e-h pairs and increasing photocatalytic efficiency is semiconductor coupling [48,49]. Combining g-C 3 N 4 with ZnO to create a multi-component system is a unique and practical method for improving light harvesting and e-h separation. The creation of a heterojunction between g-C 3 N 4 and ZnO might explain the inhibition of e-h recombination and improved harvesting of photons in the visible region [50]. The goal of the present investigation is to synthesis g-C 3 N 4 /ZnO nanocomposite via an in situ thermal decomposition to have a clean interface between these two semiconductors. Then, the photocatalytic efficiency of the produced composites was examined via MB removal. The antibacterial characteristics of samples were also assessed.
2.2. Fabrication of pristine g-C 3 N 4 and g-C 3 N 4 /ZnO nanosheets 10 grams of (CO(NH 2 ) 2 in an Al 2 O 3 crucible were heated until they were thermally decomposed at 555°C for two hours at a heating rate of 4°C min −1 to yield pure g-C 3 N 4 powder. The bulk g-C 3 N 4 was then ground into a powder and heat-treated in an electrical oven at 355°C for 3 h at the same rate. The second process is necessary for exfoliating g-C 3 N 4 powder into g-C 3 N 4 nanosheets. The g-C 3 N 4 /ZnO samples were produced in the same way, but before the second heat treatment, the g-C 3 N 4 powder was blended with different amounts of (Zn(OAC).2H 2 O.

Characterization
An x-ray Philips PW3040 diffractometer with copper radiation (=0.154 nm) was utilized to record the photocatalysts' x-ray diffraction (XRD) information. SEM (TESCAN, model MIRA III) was utilized to evaluate the morphology of specimens. TEM micrographs were taken with a CM120 microscope and a 100 kV accelerating voltage. FTIR spectroscopy was utilized to investigate the surface molecular structure (AVATAR Thermo). UV-vis spectra had been utilized with a Shimadzu UV-2450. An Avaspec 2048 TEC fluorescence spectrometer was utilized to take the photoluminescence (PL) spectra. To analyze the emission spectra, the samples were stimulated at a wavelength of 326 nm.

Photocatalytic evaluation
Under irradiation, the photodegradation of MB (10 mg/l) was generally carried out in a Pyrex beaker with 50 mg of the samples distributed in 50 ml of methylene blue solutions. The resultant specimen was blended without light for thirty minutes to have an adsorption and desorption equilibrium before being lighted. At regular intervals, three milliliters of solution were taken from the beaker, centrifuged, and examined using UV-visible spectrophotometry (Optizen 3220UV) at 664 nm (maximum UV wavelength that MB absorb).

Antibacterial properties
The antibacterial characteristics of the synthesized specimens against Escherichia coli as gram-negative and Staphylococcus aureus as gram-positive bacteria were investigated by the agar well diffusion method. The bacterial strains were purchased from Persian Type Culture Collection. The turbidity of the bacterium suspension was equal to the turbidity of the 0.5 McFarland solution. For preparation culture medium, in sterilized Petri plates, the sterilized Muller-Hinton agar solution was placed, and the plates were maintained until the medium hardened. After that, using a sterilized plastic borer, wells were drilled into the Petri plates. The germs were inoculated into the agar plates using a sterilized swab. Suspensions of 0.02 g of samples were added to the wells. Sterilized water and tetracycline were used as negative and positive controls, respectively. After that, the cultivated specimens were kept at 37°C for 24 h. A ruler was utilized to calculate the inhibition zones that had developed.

Results and discussion
The XRD arrays of pristine g-C 3 N 4 and binary nanocomposite are presented in figure 1. The peak at 13.1°is due to the (100), pertinent to the in-plane packing motif. The peak at 27.5°pertains to the (002) stacking structures (ICSD 87-1526). These peaks confirm the creation of g-C 3 N 4 [26]. The g-C 3 N 4 has a plane filling arrangement interlayer deposition structure with a period of 0.0675 nm [51,52]. Furthermore, it is found that there are diffraction peaks at 31. Scanning electron microscope images, EDS and MAP analysis were utilized to study the structure of the samples. Figures 2 and 3 present the structure of the pure g-C 3 N 4 and binary g-C 3 N 4 /ZnO composite. The images clearly reveal that the structure of g-C 3 N 4 is flat, comprising of layer morphology which is common for this material. It seems that ZnO nanoparticles cling to the g-C 3 N 4 surfaces. The EDS analyses show C and N for pure sample and C, N, Zn and O for binary specimens, which confirm the formation of g-C 3 N 4 and ZnO in the samples. MAP analyses exhibit a uniform distribution of elements which is important for the photocatalytic activity of a multicomponent semiconductor system.
TEM was employed to examine the sizes and morphologies of binary nanocomposites. The darker parts in the image may be ascribed to ZnO, while the grey region could be given to g-C 3 N 4 , as seen in figure 4. The surface and edge of the g-C 3 N 4 are decorated with ZnO nanoparticles. The interaction between the ZnO particles and the g-C 3 N 4 was strong enough that the ultrasonication procedure for dispersion of the samples on the copper gride for TEM investigation was unable to peel these nanoparticles away, implying the formation of a proper connection between two semiconductors which is critical for facile electron and hole movement among ZnO and g-C 3 N 4 .
The chemical groups of pure g-C 3 N 4 and binary nanocomposite were assessed via FTIR, as presented in figure 5. The stretching vibration associated to the O-H part of the water molecules in the specimens and the N-H vibrations of the g-C 3 N 4 edge-deficient loop is responsible for the absorption peak between 3200 and 3500 cm −1 [26]. Typical C-N or C=N have absorption peaks in the region of 1201 to 1699 cm −1 [52]. The triazine ring vibration in g-C 3 N 4 is binary C 3 N 4 /ZnO nanocomposite shows a small redshift when compared to pristine g-C 3 N 4 (811 cm −1 ), showing that g-C 3 N 4 interacts with ZnO [53]. Naturally, the interaction of g-C 3 N 4 with ZnO enhances both the development of heterojunctions and electron transport during photochemical reactions.
The effectiveness of e-h separation in the samples was determined using photoluminescence analysis. Radiative recombination of photoelectrons and generated holes is known to cause photoluminescence emissions on semiconductors [54]. It can be seen in figure 6, pristine g-C 3 N 4 presents a broad PL peak at about 461 nanometer. This peak may be attributed to the band gap emission of g-C 3 N 4 . The binary nanocomposite had much less emission intensity than pure g-C 3 N 4 . It indicates e-h recombination in g-C 3 N 4 was successfully suppressed following the development of heterojunction structures between g-C 3 N 4 and ZnO.
An UV-vis spectrometer was employed to evaluate the optical performance of all specimens (figure 7). Between 380 nm and 800 nm, the absorption peaks of the specimens were red-shifted, denoting considerably increased absorption, and the absorbance intensity increased with adding ZnO to g-C 3 N 4 . The Kubelka-Munk  formula (equation (1)) was utilized to measure the band gap energy of g-C 3 N 4 and ZnO.
and R is reflectance. E g , k, h, and u are band gap energy, a constant, Planck's constant, and light frequency, respectively. The n=0.5 for the indirect band gap and n=2 for the direct band gap. For pristine g-C 3 N 4 and binary g-C 3 N 4 /ZnO composites, the value of n is 0.5 [55]. For g-C 3 N 4 and binary composite, the extrapolated intercept in figure 8 yields E g values of 2.73 and 2.68 eV, respectively. The decreased band gap energy of the binary nanocomposite might contribute to higher visible light harvesting, promoting the production of more e − -h + pairs, and therefore increased photoactivity [56]. Figure 9 depicts the photodegradation results of the pristine g-C 3 N 4 and composites with different amounts of ZnO. Without the catalyst, no deterioration was identified under irradiation, indicating that the MB was highly stable in this situation. It takes time about 30 min to get the equilibrium. Compared to g-C 3 N 4 , the photocatalytic efficiency of the binary nanocomposite was greater under light irradiation. As can be seen, the amount of ZnO had a profound influence on the photodegradation yield of the specimens. Low loading  amounts of ZnO result in an effective decreasing interface between two semiconductors, thus decreasing the efficacy of photoinduced e-h separation and their easy transfer. On the other hand, a high amount of ZnO increases the portion of the semiconductor that works just under ultraviolet light, causing a lower efficiency of harvesting visible light. Therefore, a proper weight percent of g-C 3 N 4 to ZnO is critical to have the best photodegradation efficiency. In the present study, the g-C 3 N 4 −15 wt% ZnO (ZnO:g-C 3 N 4 weight ratio of 0.18:1) had the highest photodegradation efficiency. The photodegradation efficiency after 90 min irradiation was 25 and 73% for pure g-C 3 N 4 and g-C 3 N 4 −15wt.%ZnO, respectively. This result might be due to the proper heterojunction structures in the binary composite samples, which leads to minimal charge barrier recombination and adequate active sites. The photocatalytic process under irradiation was fitted to pseudo-firstorder kinetics, as presented in figure 10, with the reaction rate constant (k) estimated using the famous rate law equation (equation (2)) [57].
where C and C 0 depict the concentrations of MB solutions at t and t 0 , respectively. The highest k (∼0.014 min −1 ) is related to g-C 3 N 4 −15wt.%ZnO sample and the lowest k (∼0.004 min −1 ) is pertinent to pure g-C 3 N 4 . These  For effective use of photocatalysts, the catalyst's reusability is a critical factor. In other words, it is important for a photocatalyst to maintain its characteristics after several uses [58,59]. Repeated photodegradation experiments of the g-C 3 N 4 /ZnO nanocomposite were carried out to validate the catalytic life time of the synthesized samples. For this aim, the used composites were centrifuged, separated, and dried at 55°C for 24 h and then the samples were utilized for further photocatalyst experiments. Figure 11 displays the reusability of g-C 3 N 4 /ZnO nanocomposites after different cycles. The results reveal that after the several runs, there is no sharp decline in photodegradation activity. Therefore, the produced g-C 3 N 4 /ZnO nanocomposites demonstrated excellent stability during the photodegradation activity.
In the photodegradation process, organic compounds are attached to the photocatalyst and destroyed directly via charge carriers like holes or indirectly through the hydroxyl and superoxide species [60,61]. After a sequence of reactions, organic molecules are generally destroyed into water and carbon dioxide molecules. However, the development of intermediate molecules was also reported [46,62]. The photoinduced electron and hole movement in this binary system could be explained by Z-scheme or type-II mechanisms [63,64]. Trapping tests were set up to investigate the prominent mechanism in this nanocomposite, as shown in figure 12. For this aim, triethanolamine (TEA) as the hole scavenger, benzoquinone (BQ) as the superoxide scavenger, and isopropanol (IP) as the hydroxyl radical scavenger were used. As can be seen, BQ and TEA  hindered photoactivity remarkably. The results show superoxide in the CB and hole in the VB have the most decisive influence on photodegradation yield. Moreover, IP does not have much effect on photodegradation suggesting hydroxyl radicals are not mainly involved in the reactions. The oxygen to superoxide redox potential is −0.33 eV [65]. The valance band positions of g-C 3 N 4 and ZnO are −1.3 and −0.5 eV, respectively, which are favorable for the formation of superoxide species. The redox potential of ·OH to OH − is +1.99 eV [66]. The g-C 3 N 4 and ZnO have valance band positions at +1.4 and +2.7 eV, respectively. According to valance band position, the photogenerated holes on the g-C 3 N 4 could not react with H 2 O to create ·OH species, whereas ZnO could do. If the Z-scheme is a main mechanism of photocatalyst, holes must remain in the ZnO valance band and oxidize water to ·OH species. Then, ·OH radicals participate in photoreactions. However, the findings of trapping tests confirm the profound role of holes and a minor function of ·OH in the photoreactions. It shows that the type-II junction is a more probable mechanism compared to Z-scheme for this binary composite in the present work.
The photocatalytic mechanism of binary composites is postulated based on the findings of the photocatalysis investigation, as illustrated in figure 13. Electrons in g-C 3 N 4 move from the valance band (VB) into the conduction band (CB) when exposed to light, resulting in a corresponding number of holes in the valance band. In comparison to ZnO, the CB of g-C 3 N 4 is more negative. Therefore a portion of photoproduced electrons could be transported from the CB of g-C 3 N 4 to the CB of ZnO at the heterojunction contact [51,67]. The valance bond of g-C 3 N 4 has a less positive value than that of ZnO. The photoinduced holes produced in the  valance band of ZnO could move from the valance band of ZnO to g-C 3 N 4 . The photoinduced e and h eventually become separated, prolonging the lifespan of the photogenerated carriers. The photoproduced electrons combine with O 2 molecules to generate superoxide species. In addition, photoproduced holes in the solution could not interact with H 2 O and OHto generate · OH as discussed before. Organic dyes and reagents can be degraded by superoxide and hole species, as confirmed by trapping experiments which may produce CO 2 and H 2 O. Therefore, the photocatalytic efficiency of the binary composite may be enhanced via the movement of e-h between two semiconductors which results in the separation of photogenerated e and h.
Antibacterial characteristics of g-C 3 N 4 and binary nanocomposites were studied against two Gram-positive and Gram-negative strains by determining their inhibition zone, as shown in figure 14. As can be seen, the binary g-C 3 N 4 /ZnO nanocomposites with 15wt.% ZnO have higher antibacterial properties against both Staphylococcus aureus and Escherichia coli bacteria compared to pure g-C 3 N 4 specimens. Zone inhibitions against Escherichia coli bacteria for g-C 3 N 4 /ZnO, g-C 3 N 4 and positive control were 26, 18 and 19 mm, respectively. These values against Staphylococcus aureus for -C 3 N 4 /ZnO, g-C 3 N 4 and positive control were 24, 22 and 24 mm, respectively. Different possible mechanisms were reported for the antibacterial actions of nanomaterials. First, inorganic materials release ions in the environment of bacteria. For instance, it was  reported that Zn 2+ ions released from zinc oxide could attach and enter inside the cells, thus killing the bacteria [68,69]. Another mechanism is the creation of reactive oxygen groups (ROS), which may be activated in the semiconductor materials like ZnO and CuO [70][71][72]. These reactive species would destroy the bacteria membrane and damage the integrity of the bacterium. In the present work, Zn 2+ ions could increase oxidative stress in the cells and combine with the bacteria. This phenomenon could have an adverse effect on the fluidity of the cell membrane and change the normal function of bacteria [73].

Conclusions
The g-C 3 N 4 /ZnO nanocomposite was prepared and characterized. XRD and FTIR studies revealed significant coordination of ZnO with g-C 3 N 4 and minor degradation of the crystalline arrangement of pristine g-C 3 N 4 when ZnO was added. PL and DRS analyses showed that the binary nanocomposite had a lower emission intensity and better visible light harvesting compared to g-C 3 N 4 , resulting in more active sites, photoinduced e-h pairs, and lower e-h recombination. These benefits could result in a higher photocatalytic potential. According to photocatalytic test results, g-C 3 N 4 /ZnO nanocomposite showed a 3.3-fold increase in the apparent rate constant toward MB dye degradation compared to pure g-C 3 N 4 . This research presents a simple synthesis technique for making g-C 3 N 4 /ZnO nanocomposite, which has commercial potential.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).