Synthesis, Characterization, and Photocatalytic Activity of g-C 3 N 4 /GaN-ZnO Composite

Recently, photocatalysis process has shown great potential as a low-cost, environmentally friendly, and sustainable method for the water/wastewater treatment. Among that, g-C 3 N 4 is one of the most promising photocatalyst and widely used for a variety of applications. In spite of some unique features such as strong reduction ability, active under visible light, nontoxic, and high stability, g-C 3 N 4 photocatalytic capability under visible light is limited due to fast recombination rate of reactive charges. To deal with this issue, in this study, g-C 3 N 4 is combined with GaN-ZnO for reducing the recombination rate of charge carriers and increasing the active sites. The g-C 3 N 4 /GaN-ZnO composite was characterized by several methods such as SEM, EDX, XRD, FT-IR, UV-Vis, and BET. It is also observed that the composite with outstanding features can work e ﬀ ectively under visible light; thus, it is likely to be widely applied in environment treatment, especially in antibiotic residue with more than 90% of tetracycline was decomposed after 3 hours.


Introduction
The development of the industry now depends strongly on fossil fuels which will be exhausted in the future. Therefore, renewable energy sources are considered as an excellent alternative fuel source with advantages such as being available and being clean energy, without affecting the environment when exploited. Among them, solar energy is a popular renewable energy source today. The use of sunlight-based environmental treatment techniques has become one of the most potential and environmentally friendly techniques. Another concern is the water pollution caused by textile dyes, organic contaminants, and especially antibiotic residues. Antibiotics are defined as compounds or substances that destroy or inhibit the growth of bacteria. They are compounds used in animal husbandry and to prevent or to treat infections for human health; sometimes, they are used as food preservatives. During wastewater treatment, the partial or incomplete metabolism and also the ineffective removal of antibiotics have created the way for antibiotics to enter the environment including water, sediment, and soil through discharging wastewater. These pollutants have a big impact on the life of species and environmental pollution. Therefore, the removal of these pollutants is necessary. In recent years, a sustainable treatment technology by using semiconductor photocatalytic has been introduced because of various potential such as a low-cost, environmental friendly, and suitable for water/wastewater industry. The advantage of this technology is to oxidize and remove the organic compounds and microorganisms in water. Varieties of techniques have been applied to degradation those organic contaminants including many kind of photocatalyst (g-C 3 N 4 , TiO 2 , ZnO,...) which is known as one of the most promising technology [1][2][3][4][5].
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. Therefore, to achieve an ideal photocatalyst, semiconductor photocatalysts need to have a suitable band gap to utilize sufficient solar energy. The graphite carbon nitride (g-C 3 N 4 ) is known as a metal-free polymer and a conjugative π structure material. This material has many ideal properties, such as unique electric, optical, structural, and physicochemical properties; these properties already make g-C 3 N 4 -based materials become the ideal substance for catalytic and energy applications [6,7]. This is a promising nonmetallic photocatalyst in the decomposition of pollutant organic matter in visible light and the analysis of water into hydrogen and oxygen. However, the using of these photocatalyts are still faced with some drawbacks, for example, the absorption of visible light is ineffective and the quick recombination of electron-hole [8]. To improve the photocatalytic performance of g-C 3 N 4 , various studies were conducted, including nonmetallic g-C 3 N 4 doping. The doping of g-C 3 N 4 by various elements such as B, C, P, and S [2,9] has been successfully conducted; resulting in photocatalytic activity of the materials is greatly improved. The doping of g-C 3 N 4 by nonmetallic elements has become a concerned topic of research, opening up a new field of research, preparing materials that have good photocatalytic effect under visible light which meet the practical requirements. In addition to enhance the photocatalytic activity by doping a nonmetal in the lattice, the addition of a metal or metal oxide on the surface of g-C 3 N 4 has also been of interest recently. Among the metals, Ag is most concerned by the acceptable price and the ability to increase the activity of Ag nanoparticles. In this case, Ag acts both as a photosensitive agent to increase the ability to absorb visible light and to exhibit a surface plasmon effect [10]. The first SnO 2 /g-C 3 N 4 material was synthesized. The material consists of two components: g-C 3 N 4 with a low specific surface area and SnO 2 nanoparticles with a large surface area. In this composite, SnO 2 nanoparticles dispersed well into g-C 3 N 4 . The interaction between the two components is very strong; this is confirmed by the band gap energy and network parameters. The synergistic interaction between the two components in the SnO 2 /g-C 3 N 4 material is due to the contribution of the π-π conjugate effect in g-C 3 N 4 , resulting in a significant improvement in the process of electron separation, creating from the interaction between the contact surfaces and the two components. On the other hand, nanometer-sized SnO 2 particles will increase the surface area of the material, thereby increasing photon efficiency. As a consequence, this material is an active photocatalytic, greatly increasing the decomposition of methyl orange (MO) under visible light irradiation. Optimum optical efficiency of up to >90% indicates that photocatalytic capacity is much higher than separate g-C 3 N 4 and SnO 2 components [11]. Therefore, the modification of g-C 3 N 4 to create new material with high photocatalytic efficiency is necessary.
Besides, GaN-ZnO solid solution is a homogeneous phase with a lattice structure, consisting of two GaN and ZnO members dissolved in solid-state. Normally, GaN is a solvent (with a higher content), while ZnO is a solute (with a lower content). In the following years, many of these scientists' works were also published in world-renowned journals on the separation of GaN-ZnO solid solutions. Since 2005, Domen [12] and colleagues have published a large number of works on GaN-ZnO solid solutions in world-famous magazines. Initially, the authors successfully synthesized the nano-sized RuO2-doped GaN-ZnO solid solution and used it as a photocatalyst material to effectively separate water in visible light. This is one of the first publications on analyzing water under visible light on an oxynitride photocatalyst as a GaN-ZnO solid solution doped with nano-sized RuO 2 . In contrast to conventional nonoxide photocatalysts, such as CdS, the GaN-ZnO solid solution is a fairly stable material in the photocatalytic reaction [12]. In another study [3], the author showed nano-dispersed Rh-/-Cr 2 O 3 particles (core/shell) on GaN-ZnO as a catalyst for separation of pure water. Rh-Cr 2 O 3 -/-GaN-ZnO material has good photocatalytic ability in visible light area. Cr 2 O 3 shell prevents the formation of H 2 and O 2 from water on Rh nanoparticles, allowing to destroy the chemical balance of the water separation reaction, whereby the photocatalytic reaction efficiency increases significantly. The core/shell structure enhances H 2 release compared to bare Rh nanoparticles. From these studies, it can be seen that the GaN-ZnO solid solution has a special and very attractive property in the field of photocatalyst. However, concentrated works are applied in the field of water separation to produce hydrogen. Although there are many advantages in photocatalysis, the solid solution of GaN-ZnO is still rarely applied in the treatment of toxic organic substances in water. Therefore, the task of the research is to study the modification of this material to treat organic matter polluting the water environment. This is not only of scientific significance but also of high practical value.
Recently, the combination of two types of materials is GaN-ZnO and graphite carbon nitrite g-C 3 N 4 have been tested as photocatalysts [13]. Compared to other photocatalysts, GaN-ZnO and g-C 3 N 4 have many advantages such as low band gap energy, activating in visible light, high surface area, and durable and can be synthesized in large quantities. In this paper, the g-C 3 N 4 /GaN-ZnO composite photocatalyst was synthesized by a simple method and has shown a significantly improved photocatalytic activity for tetracycline (TC) degradation.

Sample Preparation.
All chemicals used in this work were analytical grade without further purification from Merck, Germany. Deionized (DI) water was used for the preparation of all required solutions.
The urea was calcinated to obtain g-C 3 N 4 . 5 g of urea was put in a crucible, sealed with aluminum foil (to prevent the sublimation of precursors as well as enhance the condensation to form g-C 3 N 4 ), then place the crucible into the furnace. The samples were heated at temperatures of 550°C in 3 hours. Finally, the furnace was naturally cooled to room temperature. The obtained polymer g-C 3 N 4 is a light yellow powder with different color intensity depending on the heat regime. The samples of synthetic g-C 3 N 4 materials are denoted g-C 3 N 4 -T, where T is the heating temperature (550°C).
The GaN-ZnO powder sample was synthesized by adding 2 g of Ga 2 O 3 and ZnO with molar ratio of 1 : 1 and 2 g of urea into an agate mortar and grind finely. The mixture was put into a porcelain boat, placed a ceramic boat in the middle of the quartz tube, and then put into a horizontal furnace. The mixture was calcinated in a stream of argon gas at temperature of 900°C for 4 hours. The products are denoted GaN-ZnO-T, where T is the heating temperature (900°C).
The composite of g-C 3 N 4 /GaN-ZnO was synthesized by adding 0.15 g of GaN-ZnO synthesized sample above and 1.05 g of urea into an agate mortar and grind finely. Then, the mixture was put into a crucible, covered with layers of aluminum foil, and put into a furnace. The sample was calcinated at 550°C for 3 hours.

2.2.
Characterization. X-ray diffractometer (XRD, Rigaku Miniflex II, Japan) is used to study the crystal structure, evaluate the crystallization level, and detect the strange crystal phase of the material. Fourier-transform infrared spectroscopy (FTIR) (Jasco 6100, Japan) is used for characterizing chemical bonds of materials. Energy dispersive X-ray (EDX) is to analyze the chemical elements of materials from Jeol EDS System. The image of sample surface is recorded by Scanning Electron Microscope (SEM) (Jeol JSM 7500FA, Japan), the signals used by a scanning electron microscope to produce an image result from interactions of the electron beam with atoms at various depths within the sample. Brunauer-Emmett-Teller (BET) (Jeol, Japan) is used to determine the surface area and the pore volume, and Diffuse Reflectance UV-vis Spectrum (UV-Vis-DRS) is to determine the band gap energy of materials.

Photocatalytic Activity Evaluation.
Photocatalytic activities of g-C 3 N 4 /GaN-ZnO samples were evaluated by TC degradation in aqueous solution under visible light of 60 W lamp irradiation. TC stock solution (1000 mg/L) was prepared weekly using distilled deionized water and stored in dark at 4°C. The pH is adjusted by 1 mol/L of H 2 SO 4 or NaOH, and C 0 and Ct are the concentrations of TC before and after photocatalytic reaction. The degradation of TC is considered through the decrease in its concentration versus irradiation time. The sample solutions were taken after 30 minutes. The TC solution obtained at times is diluted to the appropriate concentration, then measured based on the calibration curve to deduce the concentration of TC. They were analyzed using UV-Visible spectrophotometer with detection at 357 nm to take absorbance and from that take their concentration based on a calibration curve.
The removal efficiency of TC concentrations was calculated according to the differences between the initial and final concentrations. All the experiments were carried out at least three times, and the average values were calculated. The error between two experiments will not be greater than 10%.

Results and Discussions
3.1. Catalyst Characterization. The XRD patterns of g-C 3 N 4 , GaN-ZnO, and their synthesized composites are shown in Figure 1. The characteristic peaks appeared at 13.2°and 27.3°correspond to interlayer stacking of aromatic segments and tri-striazine units of pure g-C 3 N 4 sample assigned to (100) and (002) planes [13,14].  [13,15]. Between peak of (002) and (101), there also appeared one extra strange peak thought to be of ZnGa 2 O 4 spinene.
IR spectra of the composites are showed in Figure 2. The IR spectrum appears almost all peaks of g-C 3 N 4 and GaN-ZnO. At 808 cm -1 and some peaks in the range of 1250-1650 cm -1 are typical peaks for valence fluctuations of C-N bonds inside and outside the aromatic ring and the oscillation of the C=N bond. The broad absorption bands at    Journal of Nanomaterials 3332.39-3186.8 cm -1 are oscillations of secondary and primary amines. These results are similar to previous reports [1,11,16]. When comparing the spectra of the composite and GaN-ZnO material, a wide absorption band at the number of waves above 3426 cm -1 and about 1638 cm -1 is the oscillation of -OH of physical absorbing water [15,17]. The peak at the number of waves 494.6 cm -1 is the valence oscillation of the Zn-O bond [18]. In particular, the peak at 577 cm -1 of the solid solution featured for the bond of Ga-N was also discovered [17]. This proves that there are g-C 3 N 4 and GaN-ZnO in the composite model. Figure 3 shows the SEM images of GaN-ZnO and g-C 3 N 4 /GaN-ZnO material samples. The pure g-C 3 N 4 catalyst sample is mainly in the form of low-porosity blocks. The GaN-ZnO samples had a clear shape of particles, while the g-C 3 N 4 /GaN-ZnO composite materials were blurred and no clear delineation between particles; the particles are covered by a thin film, which is thought to be of g-C 3 N 4 . The size of   Journal of Nanomaterials the spherical particles is approximately 200 nm which is beneficial to enhance the photocatalytic efficiency of the synthesized material. Figure 4. This obtained result shows that the Ga, N, Zn, and O are detected with the percentage of mass of 30.65%, 6.00%, 7.99%, and 4.00%, respectively, while the element N content is still very low in the GaN-ZnO material sample. However, elemental element N significantly increased in the composite g-C 3 N 4 /GaN-ZnO material (21.64%). Figure 4(c) also indicates the appearance of C in the composite sample with the proportion of mass is 11.04%. This proves that there is an addition of element N into composite materials; the presence of element C in this spectrum further confirms the formation of composite materials from two components, g-C 3 N 4 and GaN-ZnO. In particular, in this composite material sample, there is no presence of elements other than the constituent elements from the precursor materials. This further illustrates that the sample is not containing impurity, and the formation of composites is very good.

Elemental composition of g-C 3 N 4 /GaN-ZnO material samples characterized by EDX method is shown in
In addition, the optical properties of the material are determined by UV-Vis-DRS spectra; the results are shown in Figure 5. It can be noticed that the absorption peak and the light absorption edge of all materials are located in the visible light region (Vis region). From the results of UV-Vis-DRS spectroscopy, the band gap energy of GaN-ZnO and g-C 3 N 4 /GaN-ZnO composite was also determined in Figure 5(b). The band gap energy of GaN-ZnO and g-C 3 N 4 /GaN-ZnO composite was determined to be 2.68 eV and 2.31 eV. The band gap energy of g-C 3 N 4 is 2.70 eV as reported in previous articles. Thus, the band gap energy of g-C 3 N 4 /GaN-ZnO composite is significantly lower than that of g-C 3 N 4 and GaN-ZnO materials.
The BET surface areas and porous structures of g-C 3 N 4 , GaN-ZnO, and composite g-C 3 N 4 /GaN-ZnO are showed in Figure 6. In this figure, a type IV adsorption isotherm of the composite sample was showed with a hysteresis loop in the range (P/P0) of 0.6-1.0. The surface area and pore volume (Table 1)   5 Journal of Nanomaterials greater than GaN-ZnO and composite g-C 3 N 4 /GaN-ZnO (2.787 m 2 /g, 0.014 cm 2 /g and 9.515 m 2 /g, and 0.075 cm 2 /g, respectively). In the comparison of g-C 3 N 4 , the specific sur-face area of composite g-C 3 N 4 /GaN-ZnO was decreased, which is believed due to the presence of GaN-ZnO layers on the g-C 3 N 4 surface. However, the surface area of composite g-C 3 N 4 /GaN-ZnO is not a main factor. Therefore, the improved photocatalytic performance is not influenced by the specific surface area. The increasing of photocatalytic activity of the composite materials is attributed to the more efficient separation of the photo-generated electron-hole pairs.
3.2. Photocatalytic Activity. In order to evaluate the effect of catalyst dosage, initial concentration, and initial pH on the removal efficiency, TC degradation experiments were conducted in various catalyst g-C 3 N 4 /GaN-ZnO weight (0.01, 0.02, and 0.05 g) and concentrations of TC (10, 20, and 40 mg/L) in the range of pH 5-11. Figure 7 shows the effect of catalyst dosage, initial concentration, and pH on the    Journal of Nanomaterials removal efficiency of TC. After 30 min in the dark condition, only 5% of TC was absorbed on the surface of synthesized catalyst.
The results showed that the removal efficiency increased with the increasing of catalyst weight (Figure 7(a)). After 180 min of reaction time, 94.3% of TC was degraded by using appearance by using 0.05 g of catalyst. This result reflects the high removal efficiency at higher amount of catalyst. When the number of material particles increase, there are an increasing the number of photon and dye molecules absorbed. Therefore, the surface area of nanoparticles provides more active site to produce radicals for the reaction.
In Figure 7(b), the irradiation of TC solution with the initial concentrations of 10, 20, and 40 mg/L for 180 min leads to degradation of 94.5, 48.3, and 12.1% of TC. The degradation efficiency of TC decreased with the increasing initial concentration of the interest solution. These results can come from the reason that at higher concentrations, the concentration of intermediate products increases, and the hydroxyl radicals become the limiting reactant which leads to the decrease in degradation rate constant. Besides, when increasing the TC concentration, the TC molecules also increase which can cause the excessive adsorbed molecules into the surface of the material and the lack of contact between TC molecules and the photo-generated holes or hydroxyl radicals [19]. Therefore, the TC removal efficiency is decreased in case of increasing the initial concentration of pollutants.
One of the most important parameters in photocatalytic is the effect of pH. The change of the pH in a solution affects the charge properties of the material surface as well as regulates the ionization state of the catalyst surface. Therefore, pH influences the decomposition and adsorption capacity of organic compounds. The results in Figure 7(c) showed that the removal efficiency was most effective at pH of 7. At the TC concentration of 10 mg/L, irradiation of TC solution with the initial pH of 5, 7, 9, and 11 for 180 min leads to degrada-tion of 67.2, 94.3, 60.4, and 40.7% of TC. These results are also explained by point zero charge of material (Pzc). The Pzc of the composite is determined at 6.9. To determine point of zero charge of g-C 3 N 4 /GaN-ZnO sample, the plot of delta pH value versus initial pH value was performed. The curve of this plot cut the horizontal axis at any pH which is the point zero charge of material. The obtained results were shown in Figure 8. If pH < Pzc, the positive charge of the g-C 3 N 4 surface increases, and if pH > Pzc, the negative charge of the material surface increases. At acidic or alkaline, both the material surface and TC molecule are positive charge or negative charge. Therefore, the adsorption of TC on the surface of the material decreased, and the degradation will be more effective at neutral pH. Figure 7(d) shows the TC degradation efficiency of g-C 3 N 4 , GaN-ZnO, and g-C 3 N 4 /GaN-ZnO materials. The synthesized g-C 3 N 4 /GaN-ZnO composite material showed highest degradation TC efficiency (95%) at pH = 7 (TC concentration = 10 mg/L) while the g-C 3 N 4 and GaN-ZnO showed lower treatment rate with the removal efficiency of 38.1% and 20.5%, respectively. This result indicated that the synthesized g-C 3 N 4 /GaN-ZnO composite material can be utilized for tetracycline in the environment.
To assess the repeatability of the photocatalytic activity of composite sample, the cycling test was performed by doing several cycle experiments of TC degradation using g-C 3 N 4 /GaN-ZnO composite. The cycle time is 180 min. After collecting the composite samples from the previous step, the photocatalyst was filtered and dried at 105°C for 24 hours to use for the next cycle. As shown in Figure 9, nearly 90% of TC is still degraded by g-C 3 N 4 /GaN-ZnO composite after running 3 cycles, indicating quite good stability for photocatalytic degradation for TC.
The good repeatability of photocatalytic activity of g-C 3 N 4 /GaN-ZnO composite makes this material reusable many times when applied in practice.  The TC decomposition mechanism by synthesized g-C 3 N 4 /GaN-ZnO composite material is proposed in Figure 10. In this figure, after receiving visible light, the electron-hole separation occurs simultaneously on g-C 3 N 4 and GaN-ZnO material; the electron moves to the conduction band (CB) and leaving the holes h + on the valence band (VB). In the conduction band, electrons from g-C 3 N 4 will move to the GaN-ZnO of the composite, while in the valence band, the hole from GaN-ZnO will move to g-C 3 N 4 . This process significantly reduces electron-hole recombination occurring in composites. The reduction will occur in the conduction band of GaN-ZnO, and the oxidation will occur in the valence band of g-C 3 N 4 .
The following equations were used to determine conduction and valence band potentials of the synthesized samples.
For g-C 3 N 4 /GaN-ZnO composites, g-C 3 N 4 acts as a photosensitive agent, improving the ability to absorb visible light of composite materials. With this proposal mechanism, the photocatalytic activity of g-C 3 N 4 /GaN-ZnO composites was significantly improved compared to the separate semiconductors g-C 3 N 4 and GaN-ZnO.
To further prove the possible photocatalytic mechanism, the PL spectra of g-C 3 N 4 /GaN-ZnO composite were obtained. As shown in Figure 11, the PL spectra of g-C 3 N 4 /GaN-ZnO composite were obtained with an excitation wavelength of 365 nm at room temperature. It can be seen from this figure that GaN-ZnO sample had much higher PL intensity than g-C 3 N 4 /GaN-ZnO composite which can be explained due to the higher recombination rate of charge carriers. In contrast, the intensity of g-C 3 N 4 /GaN-ZnO composite emission peak was decreased significantly, which illustrated that the recombination of the photo-generated carriers was reduced. The PL results have further confirmed the previous discussion on the photocatalytic mechanism and photocatalytic activity. The efficiently separated electrons and holes will greatly contribute to the photocatalytic reaction.

Conclusion
The novel g-C 3 N 4 /GaN-ZnO composites were successfully prepared by a simple calcination method. According to XRD pattern, IR spectrum, EDX, BET, UV-Vis-DRS, and SEM images, the presence of g-C 3 N 4 and GaN-ZnO in the composite was observed. The resulting g-C 3 N 4 /GaN-ZnO composites showed an efficiency photocatalytic activity for   Journal of Nanomaterials degradation of TC under visible light irradiation. Under the optimum experiment, the TC degradation of g-C 3 N 4 /GaN-ZnO is much higher than g-C 3 N 4 and GaN-ZnO. 95% of TC was degraded by composite g-C 3 N 4 /GaN-ZnO in comparison with the degradation of 20-30% of TC by g-C 3 N 4 and GaN-ZnO. Based on these findings, the g-C 3 N 4 /GaN-ZnO composites can be used as an alternative photocatalytic material for TC degradation in aqueous solutions or wastewater.

Data Availability
Data is available on request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.