Promoting Spatial Charge Transfer of ZrO2 Nanoparticles: Embedded on Layered MoS2/g-C3N4 Nanocomposites for Visible-Light-Induced Photocatalytic Removal of Tetracycline.

Photocatalytic degradation is a sustainable technique for reducing the environmental hazards created by the overuse of antibiotics in the food and pharmaceutical industries. Herein, a layer of MoS2/g-C3N4 nanocomposite is introduced to zirconium oxide (ZrO2) nanoparticles to form a "particle-embedded-layered" structure. Thus, a narrow band gap (2.8 eV) starts developing, deliberated as a core photodegradation component. Under optimization, a high photocatalytic activity of 20 mg/L TC at pH 3 with ZrO2@MoS2/g-C3N4 nanocomposite was achieved with 94.8% photocatalytic degradation in 90 min. A photocatalytic degradation rate constant of 0.0230 min-1 is determined, which is 2.3 times greater than the rate constant for bare ZrO2 NPs. The superior photocatalytic activity of ZrO2@MoS2/g-C3N4 is due to the dual charge-transfer channel between the MoS2/g-C3N4 and ZrO2 nanoparticles, which promotes the formation of photogenerated e-/h+ pairs. Charge recombination produces many free electron-hole pairs, which aid photocatalyst reactions by producing superoxide and hydroxyl radicals via electron-hole pair generation. The possible mechanistic routes for TC were investigated in-depth, as pointed out by the liquid chromatography-mass spectrometry (LC-MS) investigation. Overall, this work shows that photocatalysis is a feasible sorbent approach for environmental antibiotic wastewater treatment.

Introduction

In the past decade, antibiotics- and antiphlogistics-related pharmaceutical wastewater pollution has posed a hazard to human health and the environment. Tetracycline (TC) hydrochloride, a type of TC in general antibiotics, is mainly used in medicine, agriculture, and other fields, and it remains in the soil and groundwater. Improvements in TC can lead to the proliferation of drug-resistant microbes if they are exploited.[1−3] Currently, many techniques have been developed for the removal of antibiotics from water environments, the most notable of which are photo-Fenton treatment and biological treatments such as ozonation and membrane filtration. Other techniques include electrochemical oxidation, semiconductor photocatalysis, and adsorption.[4−6] Parallel to these classic technologies, semiconductor photocatalysis has gained significant attention in the field of antibiotic degradation due to its high efficiency and long-term sustainability, which is achieved through the use of solar light and ecologically favorable circumstances. Besides that, it can efficiently digest antibiotics and convert them into readily biodegradable composites with fewer harmful organic or inorganic compounds, diminishing or eliminating their antimicrobial effectiveness. It may be able to alleviate some of the issues associated with some orthodox techniques of antibiotic degradation, such as the problematic biodegradability of antibiotics and the possibility of secondary contamination caused by the intermediates.[7−10]

The most often utilized and investigated materials in heterogeneous photocatalysis are transition-metal oxides and wide-band semiconductors, such as TiO2, SnO2, CeO2, and ZrO2. In particular, the remarkable physicochemical stability of ZrO2 and the unique electronic energy band structure of this photocatalyst have piqued the curiosity of a large number of researchers. Because of their large band gap (>5 eV), ZrO2 photocatalysts can only absorb ultraviolet light, which represents a small percentage of the solar spectrum (less than 5%). As a result, the photocatalytic activity of ZrO2 is limited in its functional application as a photocatalyst. Apart from that, the poor separation rate between the charged particles generated by photons in ZrO2 restricts the photocatalytic degradation activity of the nanomaterial.[11] Among the several approaches for overturning the enhancing light-harvesting and recombination of photogenerated carriers, coupling ZrO2 with another narrow-band-gap semiconductor, for example, TiO2, TiO2–ZrO2, g-C3N4, MoS2, etc.[12−14]

Two-dimensional (2D) layered semiconductors hold several remarkable characteristics, including rapid charge-carrier detachment, electronic conduction, and a vast surface area. It has been discovered that g-C3N4, a nonmetallic polymeric nanomaterial with optimal band-edge position (1.32 V, pH = 7) and a low band gap (2.8 eV), features moderate van der Waals forces between layers and low hydrogen-bonding connectivity bordered by polymeric melon units but strong covalent C–N bonds inside the melon units.[15] g-C3N4 nanosheets can also help create nanocomposites with rich coupling heterointerfaces and surface-reactive positions. Studies on creating heterojunctions, including g-C3N4/ZrO2,[16] TiOF2/g-C3N4,[17] MoS2/g-C3N4/Bi24O31Cl10,[18] and CsPbI3/g-C3N4,[19] with the goal of overcoming the inherent disadvantages of g-C3N4, were conducted in-depth. This scheme reveals superior visible-light photoinduced activity in the direction of degradation compared to the solitary components.

Individual sandwiched S–Mo–S layers in molybdenum disulfide (MoS2) are intrinsic n-type photocatalysts with a narrow band gap (1.29–1.94 eV) and an anisotropic lamellar structure with weak van der Waals interactions between them. Molybdenum disulfide (MoS2) is a high-productivity cocatalyst for photocatalytic degradation due to the survival of unsaturated Mo and S atoms at the exposed edges, which are capable of encoring photocatalytic degradation. It also enhances visible-light absorption and minimizes reflection, allowing more free charge-transfer carriers for photocatalytic degradation.[20,21] The layered structures of MoS2 and g-C3N4 help reduce lattice disparity and help form an electronic field at the interface of a 2D–2D nanocomposite, which aids in promoting charge separation and surface reactions. Thus, the nanocomposite charge separation and photoactivity degradation of the intended ZrO2@MoS2/g-C3N4 nanocomposite should be significantly enhanced.

In this work, as mentioned above, a feasible ultrasonic chemical technique was used for the fabrication of ZrO2@MoS2/g-C3N4 nanocomposites. Here, the ZrO2 nanoparticles (NPs) were embedded on the surface of 2D layered MoS2/g-C3N4 nanosheets (NSs). The band alignment of the composite was improved by employing a continuous multistep charge-carrier (e–/h+) transfer path rather than the standard one-step process. This study investigates the photocatalytic degradation of TC over ZrO2@MoS2/g-C3N4 using visible light. A minimum energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) exists due to the delocalized π link between the TC molecule and its connection to the −OH group, leading to the high availability for visible-light absorption. The TC’s π orbital may also create an electronic interaction with the 3d orbital of Zr4+, leading to a surface complex between TC and ZrO2. It is expected that visible-light irradiation will cause photoexcitation of the surface complex. The primary goals of this work are to confirm the presence of visible-light photodegradation of TC on ZrO2@MoS2/g-C3N4 and estimate the mechanism of photodegradation. Dependence on the ZrO2@MoS2/g-C3N4 nanocomposite, strong interface effects, relatively short charge-diffusion distance, and numerous close contact interfaces can be obtained simultaneously. The possible mechanistic routes for TC were investigated in-depth, as indicated by LC–MS analysis on the ZrO2@MoS2/g-C3N4 nanocomposites.

Results and Discussion

XRD patterns of the as-synthesized products were obtained to confirm the crystal structure and the phase purity. As shown in Figure a,b, bare ZrO2 can be coded for tetragonal (JCPDS card 79-1769); its fundamental diffraction patterns at 24.2, 28.2, 31.5, 34.1, 35.3, 40.7, 50.1, and 55.3° can be assigned to (011), (111), (111), (002), (211), (202), (013), and (131) planes, respectively. The firm and spire diffraction peaks of ZrO2 confirm the high purity and crystallinity of the sample.[22,23] The primary peaks of bare g-C3N4 are located at 13.1 and 27.4°, respectively, and correspond to the crystal planes (100) and (002).[24] It has been observed that there are no distinctive peaks of g-C3N4 and MoS2 in the ZrO2@MoS2/g-C3N4 nanocomposite (Figure a,b) even though these peaks can be seen in the transmission electron microscopy (TEM) pictures. According to the results obtained for blank MoS2, the sequences of the diffraction pattern situated at 14.2, 33.2, and 58.9° are ascribable to the diffractions of (002), (100), and (110), corresponding to the usual hexagonal MoS2 structure (JCPDS card 37-1492). Remarkably, the addition of g-C3N4 and MoS2 had no adverse influence on the structure and purity of ZrO2 nanoparticles. However, the intensity of deflection points of ZrO2@MoS2/g-C3N4 is weaker and broader than that of pure ZrO2 due to the low crystallinity and the tiny crystallite size of the ZrO2 nanoparticles.

Figure 1

XRD patterns of (a) ZrO2, ZrO2@g-C3N4, ZrO2@MoS2/g-C3N4 and (b) MoS2, g-C3N4, MoS2/g-C3N4 nanocomposites.

Figure 5

HR-TEM images of (a, b) different magnifications and (c, d) SAED patterns and lattice fringes of the ZrO2@MoS2/g-C3N4 nanocomposite.

FT-IR analysis was performed to investigate the surface chemical structure of as-prepared materials. As shown in Figure , all of the as-prepared ZrO2, ZrO2@g-C3N4, and ZrO2@MoS2/g-C3N4 nanocomposites materials retain not only the surface chemical structure of the ZrO2 nanoparticles but also the structure of the g-C3N4 semiconductor. These −NH2 stretching vibrations are responsible for the wide absorption bands in the 3178 cm–1 range. The peaks show the stretching vibration of C–N at 1243 and 1404 cm–1. The peak at 806 cm–1 corresponds to tri-s-triazine stretching vibrations. The bands at 507 and 587 cm–1 as well as 780 cm–1 correspond to the stretching vibration of the Zr–O bond.[25]

Figure 2

FT-IR spectra of ZrO2, ZrO2@g-C3N4, and ZrO2@MoS2/g-C3N4 nanocomposites.

Using HR-SEM, the surface morphologies and structural characteristics of ZrO2, g-C3N4, MoS2, and ZrO2@MoS2/g-C3N4 nanocomposite were studied. Figure a depicts the irregular spherical nanoparticles of ZrO2 with an average diameter of 15 nm, resulting in a large surface area of 43.2 m2 g–1. Due to its unique spherical structure, g-C3N4 has a small surface area of 13.5 m2 g–1 compared to pure ZrO2.[26] A shape-free architecture is illustrated in Figure b, which has a smooth surface and is in the shape of a platelike structure composed of g-C3N4. It also reveals the strong interaction between the two ZrO2 and g-C3N4 semiconductors. Figure c clearly illustrates that pure MoS2 displays a hierarchical-like sheet with adjacent sizes of a limited micrometer and was piled sequentially.[27] As shown in Figure d, the ZrO2@MoS2/g-C3N4 structure is not smooth and has grooves because this pattern is formed when ZrO2 nanoparticles are embedded, and more active sites and higher light absorption capacity might be obtained with a product including dispersed nanoparticles, which would be helpful to the formation of reactive free radicals. Figure a–f shows evenly distributed Zr, O, C, N, Mo, and S in ZrO2@MoS2/g-C3N4 with a “particle-embedded-layered” structure, demonstrating that the nanocomposite of ZrO2@MoS2/g-C3N4 was successfully constructed. The results are in line with the HR-SEM analysis.[28]

Figure 3

HR-SEM images of (a) ZrO2, (b) g-C3N4, (c) MoS2, and (d) ZrO2@MoS2/g-C3N4 nanocomposites.

Figure 4

(a–f) Corresponding EDS mapping images of the ZrO2@MoS2/g-C3N4 nanocomposite.

The impact of inserting a 2D cocatalyst layered on recognition of nanocomposite creation using ZrO2 nanoparticles is established by HR-TEM scrutiny in Figure . The MoS2/g-C3N4 sheet in Figure a,b is embedded with ZrO2 nanoparticles with a regular diameter of 15 nm. The formation of spherical-shaped ZrO2 nanoparticles was confirmed by SAED analysis (Figure c). The XRD data show that the ZrO2@MoS2/g-C3N4 nanocomposite exhibits good crystallinity, and HR-TEM confirms the tetragonal structure of ZrO2 nanoparticles (101) (JCPDS no. 79-1769), which can be seen in Figure d. To define a single-phase growth, the lattice fringes show interplanar spacings (“d” values) of 0.35, 0.32, and 0.62 nm, which correspond to tetragonal ZrO2 (101), g-C3N4, and MoS2 (002) planes, respectively;[29−32] therefore, the evidence presented above suggests that the ZrO2@MoS2/g-C3N4 nanocomposite has formed.

The XPS analysis presented in Figure reveals changes in the chemical composition and surface electronic interaction of Zr, O, C, N, Mo, and S components in the ZrO2@MoS2/g-C3N4 nanocomposite. As shown in Figure a, the Zr 3d5/2 and Zr 3d3/2 diffraction peaks in the bare ZrO2 sample appeared to be at 181.9 and 184.3 eV, respectively.[33] ZrO2 nanoparticles were found to have Zr–O–Zr and Zr–O–H bonding, as shown by their O 1s diffraction values of 529.6 and 531.3 eV, respectively, in Figure b.[34] According to Figure c, the electronic states of graphite-like sp2 (C–C) and sp3 (N–C=N) in the ZrO2@MoS2/g-C3N4 nanocomposite are denoted by the C 1s deflection patterns at 284.8 and 288.2 eV, respectively, whereas the diffraction patterns at 286.4 eV mimicked a tiny amount of C–O.[35,36] In the ZrO2@MoS2/g-C3N4 nanocomposite, the N 1s diffraction peaks at pyridinic N (398.6 eV), pyrrolic N (399.5 eV), and graphitic N (401.0 eV) correlate to the (C–N=C), (N–(C)3), and N–H bondings, respectively, as shown in Figure d.[37] Among the prominent peaks are Mo 3d3/2 (232.4 eV) and Mo 3d5/2 (235.7 eV), corresponding to the Mo4+ and S 1s states in MoS2, respectively. As shown in Figure e,f, the formation of tiny amounts of Mo oxides with an adsorbed oxygen molecule results in a higher binding energy pattern (235.7 eV), which is apparent. However, as shown by XRD, there is no oxide phase in Mo, which does not affect the photocatalytic activity of the ZrO2@MoS2/g-C3N4 nanocomposite.[38] According to Figure g, the overall element profile in the ZrO2@MoS2/g-C3N4 nanocomposite is as follows. A considerable amount of interaction between the MoS2/g-C3N4 and the MoS2 would result in this instance. ZrO2 also affected the interfacial contact and charge transport of the material, resulting in the C–N pattern in g-C3N4 shifting to a higher binding energy in composites such as MoS2/g-C3N4 and ZrO2@MoS2/g-C3N4. ZrO2 is morphologically restricted, and the addition of MoS2/g-C3N4 prevents the object’s interface communication and charge transfer, altering XPS peaks in the Zr 3d and O 1s spectra. The Zr/O, C/N, and Mo/S ratios of the ZrO2@MoS2/g-C3N4 nanocomposite are shown in the atomic ratio analysis in Table .[39]

Figure 6

XPS spectra of the ZrO2@MoS2/g-C3N4 nanocomposite. (a) Zr 3d, (b) O 1s, (c) C 1s, (d) N 1s, (e) Mo 3d, and (f) S 2p (g) survey profiles and panels.

Table 1

Atomic Ratios of Zr, O, C, N, Mo, and S Derived from the XPS Data of ZrO2@MoS2/g-C3N4

element	Zr [atom %]	O [atom %]	C [atom %]	N [atom %]	Mo [atom %]	S [atom %]
ZrO2@MoS2/g-C3N4	3.21	36.12	29.13	31.17	0.13	0.24

It is seen that the surface area (SBET) of the photocatalyst is essential for determining the number of active sites and the charge-carrier transit distance. Figure a,b depicts the isotopes of the nitrogen absorption–desorption and hole size distribution curves of ZrO2 and ZrO2@MoS2/g-C3N4. The ZrO2 sample exhibits the H2-type hysteresis loop with type IV isotherms (P/P0 = 0.85–1.0), which confirms the constant size of the nanoparticles with a mesopore phase morphology,[40] As shown in Figure a, the ZrO2@MoS2/g-C3N4 nanocomposite shows type IV and H2 hysteresis loops. Two specimens demonstrated the presence of mesopores. According to the BET approach, the SBET values of ZrO2 and ZrO2@MoS2/g-C3N4 are 43.2 and 56.7 m2 g–1, respectively. The surface area of the ZrO2@MoS2/g-C3N4 nanocomposite is higher than that of ZrO2. To enhance the photocatalytic activity, the wide surface area of the sample provides a sizeable active site for light absorption, contaminant absorption, and photodegradation.[41] The pore size distribution of ZrO2 nanoparticles is 15 nm, while that of the composite ZrO2@MoS2/g-C3N4 is in the 10 nm range, with larger mesopores (Figure b). The porous MoS2/g-C3N4 not only served as a supporter but also prevented the ZrO2@MoS2/g-C3N4 nanocomposite from reaggregating as a result of the improved more extensive nitrogen acceptance and SBET of 56.7 m2 g–1, which was beneficial because the composite sample’s surface will absorb more visible light, thereby increasing the photocatalytic efficiency by providing more active sites.[42]

Figure 7

(a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of ZrO2 and ZrO2@MoS2/g-C3N4 nanocomposites.

To investigate the optical absorbance nature of ZrO2, g-C3N4, MoS2, MoS2/g-C3N4, ZrO2@g-C3N4, and the nanocomposite ZrO2@MoS2/g-C3N4, UV–vis diffuse reflectance spectra were obtained (Figure a). In the visible range, g-C3N4 has an absorption onset in the visible 460 nm, while bare ZrO2 is absorbed in the UV radiation wavelength range (absorption starts at 275 nm). As expected, the absorption onset of ZrO2@MoS2/g-C3N4 occurs between its constituents at ∼490 nm. Additionally, the band gap of ZrO2 (4.88 eV) is in agreement with the reported literature.[43,44] The addition of MoS2/g-C3N4 resulted in a modest modification in the Tauc plots, as illustrated in Figure b–d. As a result of the shift in absorbance edges for the nanocomposite structure, the band gaps for ZrO2@g-C3N4 and ZrO2@MoS2/g-C3N4 are 2.82 and 2.8 eV, respectively. The fact that such a modest change in band spacing supports the g-C3N4 and MoS2 sheet production with tight interface contact indicates the presence of ZrO2 nanoparticles and nanocomposite synthesis. Theoretically, the nanosize composite has a larger contact interface area, which allows for a faster charging carrier transit and, as a result, suppresses charge recombination, resulting in increased photocatalytic degradation efficiency.[45]

Figure 8

(a) UV–vis DRS spectra of ZrO2, g-C3N4, MoS2, MoS2/g-C3N4, ZrO2@g-C3N4, and ZrO2@MoS2/g-C3N4 nanocomposites, and (b–d) corresponding Tauc plots of ZrO2, ZrO2@g-C3N4, and ZrO2@MoS2/g-C3N4 nanocomposites.

The photoluminescence spectra were primarily conducted to evaluate the charge transfer and separation behaviors to analyze the starting point of the enhanced photocatalytic activity. Figure a shows that, under 325 nm laser stimulation, pure g-C3N4 exhibits an exceptionally bright photoluminescence pattern with the emission wavelength centered at about 460 nm, which can be attributed to its fast charge recombination. In the presence of ZrO2@g-C3N4 and MoS2/g-C3N4, the emission peak of the mixture is slightly lowered as a result of poor interactions between the constituents in the mechanical mix.[46] The EIS Nyquist plot provides an additional context for exploring spatial charge-transfer assets. The lower the charge-transfer resistance in the broad spectrum, the smaller the semicircle diameter of the EIS Nyquist plot as the spectrum narrows. Following Figure b, the relative arc diameters of the samples can be organized in the following ways: ZrO2 > ZrO2@g-C3N4 > MoS2/g-C3N4 > ZrO2@MoS2/g-C3N4, indicating that the nanocomposite interaction between ZrO2 and MoS2/g-C3N4 will result in the realization of a rapid and effective spatial charge separation process. The above analysis reveals that a more significant number of electrons and holes are used in the photocatalytic process in the ZrO2@MoS2/g-C3N4 nanocomposite-based reaction compared to the conventional reaction. As illustrated in Figure c, transient photocurrent measurements of ZrO2@MoS2/g-C3N4 reveal a higher photocurrent density than that of pure ZrO2, indicating a very effective separation of photoexcited charges (electron–hole pairs) and restriction of their recombination. Several repeats of cycles display a similar photocurrent reaction. Significantly, it can be seen that the samples have high photostability.[47]

Figure 9

(a) PL spectra of g-C3N4, MoS2/g-C3N4, ZrO2@g-C3N4, and ZrO2@MoS2/g-C3N4. (b) EIS Nyquist plots of ZrO2, ZrO2@g-C3N4, MoS2/g-C3N4, and ZrO2@MoS2/g-C3N4 nanocomposites. (c) Transient photocurrent response of ZrO2 and ZrO2@MoS2/g-C3N4 nanocomposites.

The recycle productivity of ZrO2@MoS2/g-C3N4 was examined by recovering the photocatalyst, and the results are given in Figure a. The degradation percentages were 94.8 and 92%, respectively, when compared to the first two recycling cycles. Photocatalytic efficiencies decreased to 91 and 90.1% for the third and fourth recycling cycles, respectively. Until the photocatalyst was used in the fourth cycle, there was little change in the degradation percentage. The absorption of the TC solution on the surface of the photocatalyst may be responsible for the slight divergence in the photodegradation during the recycling experiment. The photocatalytic process is carried out in various sacrificial agents using a ZrO2@MoS2/g-C3N4 catalyst to identify the active species that participate in TC degradation and remove specific reactive species. Continuous radical scavenging studies are being performed to evaluate the mechanism of ZrO2@MoS2/g-C3N4 for TC degradation. In the tests, BQ, triethanolamine (TEOA), and isopropanol (IPA) were used as •O2–, h+, and •OH scavengers, respectively. As illustrated in Figure b, the TC degradation efficiency of the ZrO2@MoS2/g-C3N4 nanocomposite is significantly suppressed in the presence of BQ (30%), which shows the primary quencher of hydroxyl (e–) species in the degradation reaction, compared to a hole (h+) and electron (e–) scavenger. From the absorbed oxygen molecule, photogenerated electrons (e–) form •O2– radicals, and •OH species form from the hole (h+) in the aqueous solution. In the optimal ZrO2@MoS2/g-C3N4 nanocomposite [TC = 20 mg/L at pH 3], the TOC technique was used to evaluate TC mineralization. Figure c shows that the TC mineralization efficiency was 71%, and the photocatalytic efficiency was 94.7% under 90 min of radiation. These results indicated that the ZrO2@MoS2/g-C3N4 nanocomposite presents the highest mineralization ability in the TC degradation process.[48]

Figure 10

(a) Recycle efficiency of the ZrO2@MoS2/g-C3N4 photocatalyst for four cycles. (b) Percentage of the photocatalytic degradation of TC with ZrO2@MoS2/g-C3N4 in the presence of different scavengers (TC conc.: 20 mg/L; ZrO2@MoS2/g-C3N4 50 mg; TEOA: (1.6 × 10–4 mol L–1) in 100 mL/BQ – (4.0 × 10–4 mol L–1) 100 mL/IPA – (1.0 × 10–1 mol L–1) in 100 mL, irradiation time: 90 min), (c) TOC mineralization efficiency for TC of the ZrO2@MoS2/g-C3N4 catalyst.

The photocatalytic degradation activity of all prepared photocatalysts is determined by monitoring the decomposition of TC in an aquatic solution under visible-light illumination, as illustrated in Figure . The primary blank test demonstrated that no degradation could be identified in the absence of light, indicating that the photocatalyst was the main reason for the degradation. The minor decrease in the TC content of ∼4% is due to adsorption in the blank condition. After 90 min, the photocatalytic activity of ZrO2@MoS2/g-C3N4 is the highest among them, and the degradation rate of TC when using this compound reaches 94.8%. When compared to other photocatalysts, the composite material ZrO2@MoS2/g-C3N4 was more efficient than ZrO2, g-C3N4, ZrO2@ g-C3N4, and MoS2/g-C3N4. The percentages of TC degradation for photocatalysts ZrO2, g-C3N4, ZrO2@g-C3N4, MoS2/g-C3N4, and ZrO2@MoS2/g-C3N4 are 41.9, 68.6, 79.3, 86.9, and 94.8% under visible-light irradiation in 90 min, respectively, as shown in Figure a. To determine the optimal dose of ZrO2@MoS2/g-C3N4 in TC degradation, various weight ratios, including 30, 40, 50, and 60 mg of photocatalyst, were used for photocatalytic degradation and examined under comparable test circumstances, as shown in Figure b. When comparing the various weight ratios of ZrO2@MoS2/g-C3N4, the photodegradation efficiencies of the various weight ratios were 44.6, 59.4, 94.8, and 74.4%, corresponding to the 30, 40, 50, and 60 mg weight ratios. Increasing the photocatalyst volume enhances the absorption, but increased photocatalyst density accelerates the TC degradation.[49] Under these test conditions, it is reported that 50 mg/100 mL of photocatalyst is the most efficient and optimal for the effective degradation of TC when exposed to visible-light irradiation for 90 min. With an increase in the photocatalyst concentration, the TC degradation rate increases to 50 mg/100 mL, and an increase in the photocatalyst concentration further reduces the degradation rate. When photocatalyst particles are present in quantities more than 60 mg/100 mL (50 mg/100 mL), the dispersion of light by the particles slows down the rate of degradation.[50] The ZrO2@MoS2/g-C3N4 nanocomposite dose and photocatalyst concentrations in Figure c,d show the photodegradation kinetics of TC. The pseudo-first-order rate constants are 0.0035, 0.0083, 0.0122, 0.0145, and 0.0230 min–1 (in Figure e) for ZrO2, g-C3N4, ZrO2@g-C3N4, MoS2/g-C3N4, and ZrO2@MoS2/g-C3N4, respectively. The photocatalyst weights of 30, 40, 50, and 60 mg are represented by the kinetic rate constant values of 0.0060, 0.0084, 0.0236, and 0.0140 min–1, respectively. Photocatalytic degradation occurs best under conditions of visible-light illumination, as measured by kinetic rate constants. The determined values are 0.0230 min–1 for the composite material and 0.0236 min–1 for 50 mg of the photocatalyst. According to Figure f, the absorption band’s intensity reduces in the presence of the ZrO2@MoS2/g-C3N4 catalyst with the intensity of the absorption band decreasing in proportion to the time of visible-light irradiation. The UV absorption at 357 nm decreases over time due to the decay of TC.[51] The effect of different TC concentrations (10, 20, 30, and 40 mg/L TC) on the photocatalytic degradation of TC was studied to establish the optimal state of the catalyst utilized. TC degradation increases from 55.9, 93.8, 69.6, and 79.3%, respectively. Because of the homogeneity of the catalyst and the challenge of light that enters the catalyst surface, the photocatalytic degradation reduces as the TC concentration increases to 40 mg/L. Figure a illustrates this phenomenon.[52] When a starting TC concentration of 20 mg/L was used, the photodegradation of TC by ZrO2@MoS2/g-C3N4 was studied at different pH values of 3, 5, 7, and 9, and the degradation percentage is shown in Figure b. It takes 90 min of visible-light illumination to significantly reduce the TC degradation percentages to 93.8, 88.3, 70.1, and 60.5% at pH levels of 3, 5, 7, and 9, respectively. The best performance is for TC degradation at pH levels 3 and 5. According to previous reports, TC is amphoteric, with pKa values of 3.4, 7.7, 9.8, and 12. It is primarily in the neutralized form TC0 from pH 3.4–7.7 and the deprotonated form TC– at pH 7.7–12. The charge intensity of TC increased with pH increasing, resulting in a more intense species attack against molecules.[53] The kinetic curves in Figure c,d were used to derive the rate constants for various TC concentrations. Parameters 10, 20, 30, and 40 mg/L TC and pH 3, 5, 7, and 9 are mimicked by pseudo-first-order rate constants of 0.0191, 0.0263, 0.0131, and 0.0179 min–1. The maximal operating rates for maximum decay were 0.0263 and 0.0238 min–1 at 20 mg/L and pH 3, respectively. Table lists some statistics to compare with other early reports on photodegradation of TC by composite photocatalysts.

Figure 11

Photocatalytic degradation of TC under different conditions: (a) different catalysts (catalysts: 50 mg; TC: 20 mg/L), (b) different dosages of ZrO2@MoS2/g-C3N4 [TC conc.: 20 mg/L], (c) pseudo-first-order kinetic plots of ln(C0/C) vs time for different catalysts and (d) different dosages of ZrO2@MoS2/g-C3N4, (e) kinetic constants of different catalysts (TC: 20 mg/L; catalyst dose = 50 mg), and (f) absorbance spectrum (TC: 20 mg/L; catalyst dose = 50 mg).

Figure 12

Photocatalytic degradation of TC under different concentrations: (a) different TC concentrations (catalyst dose = 50 mg), (b) different pH conditions of TC in the presence of ZrO2@MoS2/g-C3N4, (c) first-order kinetic plots of ln(C0/C) vs time for different concentrations, and (d) different pH of TC in the presence of ZrO2@MoS2/g-C3N4 (TC: 20 mg/L; catalyst dose = 50 mg).

Table 2

Comparison of Photocatalytic Efficiencies of TC Degradation for Different Photocatalysts

photocatalyst	Ccatalyst dose (mg)	CTC conc. (mg/L)	light source (Xe lamp, W)	kinetic constant (min–1)	degradation (%) (time)	ref
Fe3O4@BiOCl/BiVO4	50	20 (100 mL)	300 (λ > 420 nm)	0.0263	87 (90 min)	(2)
SiO2-Fe2O3@TiO2	10	10 (50 mL)	300 (λ > 420 nm)		80 (80 min)	(4)
g-C3N4/ZrO2	2	10 (5 mL)	300 (λ > 420 nm)	0.0474	90.6 (60 min)	(16)
MoS2/B-rGO	20	40 (100 mL)	300 (λ > 420 nm)	0.0203	85.2 (90 min)	(20)
Au/Pt/g-C3N4	100	20 (100 mL)	500 (λ > 400 nm)	0.4286	93 (180 min)	(33)
WO3/g-C3N4	50	25 (100 mL)	300 (λ > 420 nm)	0.0120	70 (120 min)	(60)
Sn3O4/g-C3N4	50	10 (100 mL)	500 (λ > 420 nm)	0.0108	72.2 (120 min)	(61)
g-C3N4/Nb2O5	100	20 (100 mL)	250 (λ > 420 nm)	0.0096	76.2 (150 min)	(62)
Bi/α-Bi2O3/g-C3N4	50	10 (50 mL)	300 (λ > 400 nm)	0.0122	91.2 (180 min)	(63)
BiOI/g-C3N4/CeO2	50	20 (30 mL)	300 (λ > 420 nm)	0.0205	91.6 (120 min)	(64)
ZrO2@MoS2/g-C3N4	50	20 (100 mL)	300 (λ > 420 nm)	0.0230	94.8 (90 min)	this work

The presence of the ZrO2@MoS2/g-C3N4 nanocomposite degraded the TC in around 90 min, as analyzed by liquid chromatography–mass spectrometry (LC–MS). LC–MS analysis clarified the photodegradation pathway of TC. The uniform mass spectrum obtained after 90 min of reaction with TCs and the resulting mass spectra are shown in Figure . Figure shows that TC was reduced to 10 primary photointermediate pathways (I and II), which are denoted as P1–P10 in the direction of the maintenance period. Additionally, based on the positive m/z ratios discovered and the findings of previous studies, the structural and chemical formulae of restricted byproducts were established, as shown in Figure . In addition, a possible photodegradation route was proposed, as revealed in Figure . The N-demethylation of TC resulted in the production of Pathway and an intermediate of P1 with an m/z of 415 (m + 1)+. Further photodegradation led to product P1 led to product P2 (m/z 279.20) (m + 2)+, which was formed due to the loss of the formamide group and subsequent oxidation, resulting in the construction of the resulting hydroxyl structure. Due to the low C–N binding energy, the carboatomic ring disintegrates. In other words, they can be attributed to the development of product P3 (m/z 195.10) (m + 1)+. Meanwhile, due to the low binding energy of the carbonyl and hydroxyl groups, the carbonyl and hydroxyl groups were removed. As shown in Figure , active groups were removed further, resulting in product P4 (m/z 163.10) (m + 1)+, the end product. Product P5 (m/z 145.25) (m – 1)+ was formed due to the dissociation of the oxidation of the hydroxyl group in product P4. In pathway II, the radicals may initially react with the C=C of TC, resulting in a composite fabrication with an m/z value of P6 386.80 (m – 2)+. In one photodegradation pathway, additional confronting of free radicals results in the cleavage of the aromatic ring at C=C, which produces the combinations with m/z values of P7 316.50, P8 209.10 (m + 1)+, P9 177.10 (m + 1)+, and P10 106.25. Finally, the fragments of this photointermediate were degraded to form CO2, NH4+, and H2O.[54−56]

Figure 13

LC–MS spectra of the TC degradation intermediate products in the presence of the ZrO2@MoS2/g-C3N4 nanocomposite photocatalyst after 90 min of irradiation time.

Figure 14

Proposed possible degradation pathway and primary intermediate photoproducts of TC in the ZrO2@MoS2/g-C3N4 nanocomposite with the combined photocatalytic system.

In a subsequent investigation, the variation of radical species in the photocatalytic mechanism of photodegradation of TC on the catalyst occurring is shown in Figure . However, the radical trapping experiments discussed above proved the existence of the element •O2– in the first place. As a result, •OH does not participate as an active species. Due to the high probability of O2/•O2–, the CBM values of ZrO2 and g-C3N4 are greater than those of O2. Therefore, O2 can condense to •O2–, and the TC is impaired due to the substantial reduction of •O2– in the atmosphere. Because of the photoinduced VB to CB electron transition in MoS2/g-C3N4, holes appear in the VBM of semiconductors in the ZrO2@MoS2/g-C3N4 system when exposed to visible light. As a result of the change in the CBM capacity, the photoinduced electrons on the surface of MoS2/g-C3N4 migrate to the CB of ZrO2, but the holes on the surface of MoS2/g-C3N4 remain on the surface as a result of the change in the CBM capacity.[57,58] Specifically, when it comes to ZrO2, the restricted band energy is high, and visible light is insufficient to attract electrons from their current valence band to their current conduction band. The absorption of visible light by g-C3N4 allows it to produce the π–π* transition and transfer the excited-state electrons from the VB to the CB. Therefore, electrons transferred from g-C3N4 to the ZrO2 conduction band will not be a barrier to the VB when they enter the bar because it is either a type II charge transfer or a type I charge transfer. The scheme can effectively develop the charge separation of photoinduced electron–hole pairs, thereby significantly reducing the chance of e–/h+ pair recombination.[59] The ZrO2@MoS2/g-C3N4 nanocomposite exhibits a superior photocatalytic activity because the scheme can effectively develop the charge separation of photoinduced electron–hole pairs, thereby significantly reducing the chance of e–/h+ pair recombination. In the ZrO2@MoS2/g-C3N4 mechanism, the overhead photoluminescence and EIS, electron–hole pair recombination, and photocatalytic efficiency all indicate that the recombination of electron–hole pairs has been curtailed, and the straight oxidation capability of holes has been dramatically increased, resulting in the presence of both h+ and •O2– as active species in the ZrO2@MoS2/g-C3N4 nanocomposite. According to the mechanism (Figure ), the contact between MoS2/g-C3N4 and ZrO2 influences the electron–hole pair separation efficiency. Due to the high quantities of ZrO2, the contact size is reduced, boosting the separation efficiency and photocatalytic efficiency. As a result, MoS2/g-C3N4 is the most effective cocatalyst in the ZrO2@MoS2/g-C3N4 nanocomposite for photocatalysts in the degradation of TC.

Figure 15

Proposed possible photocatalytic degradation mechanism of TC in the ZrO2@MoS2/g-C3N4 nanocomposite.

Conclusions

In summary, ZrO2 nanoparticles are embedded in a layered MoS2/g-C3N4 composite to form a “particle-embedded-layered” structure via a feasible ultrasonic chemical method. Under visible-light irradiation, the ZrO2@MoS2/g-C3N4 nanocomposite has proven to be a viable photocatalyst for tetracycline (TC) degradation. With an apparent kinetic rate constant κ of 0.0230 min–1, the photocatalytic degradation efficiency of TC over ZrO2@MoS2/g-C3N4 was determined to be around 94.8% in 90 min. The dual charge-transfer channel between the layers of MoS2/g-C3N4 and ZrO2 nanoparticles is the reason for the superior photocatalytic activity of ZrO2@MoS2/g-C3N4. It promotes the formation of photoinduced charge carriers while also reducing photoinduced charge recombination, resulting in more free charge carriers available to aid photocatalytic reactions via the production of the •O2– radical. To account for the removal of the tetracycline from the aqueous solution, the LC–MS measurements for the reaction intermediates, the reaction pathway, and the mechanism were also carried out. As a result, these photocatalysts may also provide feasible and long-term solutions to the environmental issues that antibiotic-polluted effluents cause.

Experimental Section

Synthesis of ZrO2 NPs

About 2.5 mmol of ZrOCl2·8H2O was ultrasonically scattered in 70 mL of deionized water. The ammonia solution was gradually added with vigorous stirring while maintaining a pH between 10 and 11. The sediments above were transferred to a Teflon-lined stainless-steel autoclave and warmed to 200 °C for 12 h before being left to cool to ambient temperature. The deposits were centrifuged and dehydrated at 80 °C after being rinsed with DI water and ethanol numerous times. The completed products were then calcined for 2 h in static air at 400 °C.

Loading ZrO2 NPs onto Layered MoS2/g-C3N4 NSs

Direct heating in melamine at 540 °C for 4 h in a furnace to obtain a graphite-like C3N4 was accomplished using a muffle furnace. A feasible ultrasonic chemical and self-assembly method (Scheme ) was used to prepare the ZrO2@MoS2/g-C3N4 nanocomposite. A total of 0.5 g of g-C3N4 powder was generally ultrasonically homogenized in 40 mL of deionized water for 2 h. The sodium molybdate (1 mmol) solution was added to the thiocarbamide (2 mmol) solution, dropped into the above light-yellow suspension, and treated hydrothermally. Following continuous washing and heat-drying, a suitable quantity of stacked MoS2/g-C3N4 and pure ZrO2 NPs (0.1 g) was scattered in 40 mL of methanol for 2 h and then violently agitated continuously in a fume cupboard to eliminate the solution. To improve the contact between the MoS2/g-C3N4 layers and the ZrO2 matrix, the products were ground and then sintered at 350 °C for 2 h under a static airflow to increase the connection between the layers.

Scheme 1

Representation of Feasible Ultrasonic Chemical and Self-Assembly Syntheses of the ZrO2@MoS2/g-C3N4 Nanocomposite

Evaluation of the Photocatalytic Activity and Active Species Capturing Experiments

Using 2 mg of TC and 50 mg of ZrO2@MoS2/g-C3N4 catalyst in 100 mL of DI water, the pH of the suspension was calibrated using diluted H2SO4 and NaOH. Without irradiation, the absorption–desorption equilibrium of the TC pollutant on ZrO2@MoS2/g-C3N4 catalysts can be obtained after 30 min of magnetic stirring in the dark. The light source was used as a visible light with a cutoff filter (420 nm ≤ λ ≤ 760 nm) at 300 W Xe. During photocatalytic irradiation, a small aliquot of 4–5 mL of suspension was taken out at 15 min intervals and centrifuged for 10 min at 5000 rpm. Finally, the concentration was calculated using a UV–vis spectrophotometer and the absorption peak value of λmax = 357 nm (TC maximum absorption wavelength).

To detect the active scavenger species, isopropyl alcohol (IPA) (1.0 × 10–1 mol L–1) was employed as a hydroxyl radical (OH•) inhibitor. Benzoquinone (BQ) (4.0 × 10–4 mol L–1) was utilized to capture the photogenerated superoxide radical (•O2–) and triethanolamine (TEOA) (1.6 × 10–4 mol L–1) to react with photogenerated holes (h+).[65−67] Preliminary tests were carried out to determine the adequate number of scavengers to use in photocatalytic testing.

LC–MS Analysis

LC–MS analysis was performed in an LC–MS 2020 system equipped with an LC10ADVP binary pump (Shimadzu, Japan). The sample was separated in a Phenomenex column (250 × 4.6 mm2, 5 μm) using acetonitrile (B)/water [(A) (0.1% formic acid)] as the mobile gradient phase. The injection volume was 20 μL, and the flow rate was set at 0.8 mL/min. Detection was done at a wavelength (λ) of 280 nm, with a run time of 20 min. The mass (MS) compartment consisted of a single quadrupole mass spectrometer with an electrospray ionization (ESI) source, and nitrogen gas was used to assist with nebulization at a flow rate of 1.5 L/min. The temperature was set for a curved desolation line (CDL) and heat blocks at 250 and 280 °C. All of the data were collected and processed using Lab Solution software (Shimadzu).

Analytical Characterization

This study examined the as-prepared materials using a PAN analytical X’pert pro-X-ray diffractometer equipped with a Cu K radiation (=1.54 Å) source at 40 kV and 40 mA at a temperature range of 2 = 5–70° using X-ray diffraction (XRD) patterns. With the support of Agilent Technologies, we were able to acquire the Fourier transform infrared (FT-IR) spectra of the solid sample embedded in the KBr pellets while the sample was still at room temperature. HR-TEM (200 keV, JEOL, JEM-2100f) and HR-SEM (America, FEI: NOVA Nano SEM 450) were employed to examine the morphology and microstructure of the structural applications. X-ray spectroscopy (XPS, EDXS, and ISIS300 Oxford) maps the ZrO2@MoS2/g-C3N4 nanocomposite surface profiles. The XPS binding energy data were calibrated using the carbon peak as a reference. The surface area and textural features were determined using an Autosorb IQ (quantachrome instruments version 5.0) and were assessed using the Brunauer–Emmett–Teller (BET) isotherm. The Barrett–Joyner–Halenda (BJH) method was used to determine pore size distributions. The catalyst’s diffuse reflectance spectroscopy (DRS) was carried out at room temperature using a Shimadzu UV 3600 plus in the 200–800 nm wavelength range to determine its UV–vis reflectance. PL measurements were taken with a Fluorolog (Horiba Yvon) spectrophotometer and recorded on a microfilm strip. A three-electrode setup was used to investigate the EIS response of all produced samples (Shanghai Chenhua CHI-660D). The photocatalytic performance evaluation was carried out with the help of a lamp source (Xe lamp, 300 (420 nm)). We used LC–MS analysis in the LC–MS 2020 system equipped with an LC10ADVP binary pump (Shimadzu, Japan).