Efficient photocatalytic degradation of organic pollutants over TiO2 nanoparticles modified with nitrogen and MoS2 under visible light irradiation

Investigate the use of visible light to improve photocatalytic degradation of organic pollutants in wastewater. Nitrogen-doped titania and molybdenum sulfide nanocomposites (NTM NCs) with different weight ratios of MoS2 (1, 2, and 3 wt.%) synthesized by a solid state method applied to the photodegradation of methylene blue(MB) under visible light irradiation. The synthesized NTM composites were characterized by SEM, TEM, XRD, FT-IR, UV–Vis, DRS and PL spectroscopy. The results showed enhanced activity of NTM hybrid nanocrystals in oxidizing MB in water under visible light irradiation compared to pure TiO2. The photocatalytic performance of NTM samples increased with MoS2 content. The results show that the photodegradation efficiency of the TiO2 compound improved from 13 to 82% in the presence of N-TiO2 and to 99% in the presence of MoS2 containing N-TiO2, which is 7.61 times higher than that of TiO2. Optical characterization results show enhanced nanocomposite absorption in the visible region with long lifetimes between e/h+ at optimal N-TiO2/MoS2 (NTM2) ratio. Reusable experiments indicated that the prepared NTM NCs photocatalysts were stable during MB photodegradation and had practical applications for environmental remediation.

www.nature.com/scientificreports/ electronhole pairs, but the fast recombination rate of electron-hole pairs in TiO 2 limits its application in photocatalysis 21 .To extend the lifetime of photogenerated electron-hole pairs, hybrid photocatalysts composed of semiconductor hetero-junctions should suppress the fast recombination rate of photogenerated charge carriers 22,23 . In this regard, many efforts have been made to reduce the band gap and improve its photocatalytic activity 24,25 . A recognized material to extend the photoresponse range to visible light is to dope TiO 2 with a non-metallic dopant, nitrogen 26,27 . The combination of TiO 2 and nitrogen at different energy levels improves the electron-hole separation efficiency and enhances the efficiency of the photocatalyst reaction. Furthermore, by combining TiO 2 with other bandgap semiconductors such as MoS 2 , it is possible to create heterogeneous photocatalysts. MoS 2 is a non-toxic, highly stable, strong oxidizing and relatively inexpensive material. Due to its large surface area, MoS 2 can act as an excellent catalyst for N-TiO 2 28,29 . MoS 2 exhibits a layer-dependent tunable bandwidth, an indirect bandwidth of 1.2 eV, a direct bandwidth of 1.9 eV, and high theoretical catalytic activity 30,31 . Due to their bandgap, semiconductors have been used in AOPs to photocatalytically degrade organic pollutants, especially those with the ability to absorb visible light.Combining N-TiO 2 and MoS 2 at different energy levels improves the efficiency of electron-hole separation and enhances the efficiency of the photocatalytic reaction 32,33 .The work presented here focuses the visible-light-driven photo degradation of a dye contaminants, specifically methylene blue (MB) dyes, into environmentally friendlyCO 2 and H 2 O. novel hetero-nanocomposite of N-TiO 2 /MoS 2 (NTM) as a photocatalyst using a solid-state method with low temperature synthesis, cost efficiency and easy control of reaction kinetics compared other methods. In addition, the physicochemical properties of the obtained samples have been extensively investigated to discover the excellent photocatalytic activity for MB decomposition under visible light radiation compared with pure TiO 2 . The synthesized NTM has proven to be an effective photocatalyst for applications in environmental protection.
Nanoparticles synthesis. Synthesis of TiO 2 nanoparticles (T). TiO 2 NPs were synthesized using sol-gel method 14 . In a typical synthesis, an appropriate amount of Ti isopropoxide precursor mixed to dis-H 2 O was dissolved in 87.5 ml of ethanol and then stirred for 4 h at room temperature, washed several times with deionized water and ethanol, and then dried in oven at 90 °C over night. Finally, the resulting powder was calcinated at 500 °C in a muffle furnace for 1 h in the air to extract the TiO 2 NPs.
Synthesis of TiO 2 &N nancomposite (NT). The sol gel method 34 was used to synthysize N&TiO 2 (NT) nanocomposite. Firstly, 10 mL TIPO was added to 40 mL ethanol and vigorously stirred at RT for 30 min (solution A). Secondly, (solution B) contains10 mL of ethanol, 10 mL of NH 4 OH solution (28 wt.%), and 2 mL HNO 3 . Then, solution A was added to solution B with solwley addaition under vigorous stirring. The obtained yellow semi-transparent sol was created after 2 h continuous stirring, then aged for 6 h at room temperature in air to form a homogeneous gel, which was dried for 36 h in an electric oven at 80 °C. Finally, the dry gel was milled into powders and calcined at 400 °C for 4 h a furnace set in air with a heating rate of 3 °C min −1 to yield NT nanocomposite.
Synthesis of MoS 2 nanoparticle (M). MoS 2 nanoparticles (M) were synthesized by a solvothermal reaction 35 . In this process, Na 2 MoO 4 (3 mmol, 0.726 g) and ethylene glycol (40 mL) were dissolved in 50 mL deionized water, then add thiourea (15 mmol, 1.1418 g). The mixed solution was sonicated for 30 min at RT, transferred to a Teflon lined stainless steel autoclave and kept at 180 °C for 12 h. After cooling to room temperature, the products were separated by centrifugation, washed three times with absolute ethanol and deionized water, and then dried at70 ℃ for 12 h. Finally, the black powder was obtained.
Experimental techniques. The morphology of prepared materials was studied by transmission electron microscope (TEM) model JEM-2100, JEOL, Japan and scanning electron microscope (SEM) (JEOL). The phase of the prepared samples was examined by X-ray diffraction (XRD) using a diffractometer (Panalytical XPERT PRO MPD). CuKα radiation (λ = 1.5418 Å) was used at 40 kV and 40 mA. The functional groups were identified using a Fourier transform infrared (FT-IR) spectrometer model Spectrum One (Perkin Elmer, USA) in the wave number range of 400-4000 cm −1 . Optical reflectance was recorded using a UV-Vis spectrometer (Perkin Elmer Lambda 1050). The photoluminescence spectra were recorded by a Cary Eclipse fluorescence spectrophotometer.
Photocatalytic activity study. The photocatalytic degradation activity was investigated using aphotoreactor with 400 W Halogen lamp as the light source. The distance between the halogen lamp and the dye solution www.nature.com/scientificreports/ is 10 cm. Then, 0.025 g of hetero-photocatalyst was added to 50 mL of 50 ppm MB dye solution and to achieve adsorption-desorption equilibrium, the solution was stirred in the dark for 30 min. The photodegradation reaction was initiated for 150 min, and 5 mL of the suspension was collected a period of 15 min. The obtained suspension was analyzed by UV-vis spectrophotometer at at MB solution maximum absorption wavelength at 664 nm. Figure 1 illustrates the SEM images of pure TiO 2 , NT, and NTM 2 nanocomposites. Figure 1a shows the SEM image of pure TiO 2 with interconnected spherical particles and a sponge-like structure, Also, the morphology of NT was appeared as spherical particles and exhibited the porous structures Fig. 1b. Apparently, the morphology of NTM 2 is shown in Fig. 1c. MoS 2 appeared as flowers shape grows uniformly on the surface of N-TiO 2 spheres. TEM images of the pure TiO 2, NT, and NTM 2 nanocomposites show a spherical shape with different grain sizes. Figure 1d shows the TEM micrograph of the TiO 2 showing that the nominal size of the TiO 2 nanoparticles is about 9 nm and that the nanoparticles appeared to be relatively homogeneous and uniform in despite being quite clustered with together. While the shape of nitrogen doped TiO 2 is more angular and slightly longer than that of the undoped TiO 2 with a grain size about 10 nm shown in Fig (002), which is in good agreement to the anatase phase (JCPDS 21-1272) 37 . XRD spectrum of NT showed that nitrogen doping restricted the conversion of anatase to brookite. Also detected a peak in (1 0 1) lattice plane of anatase TiO 2 shift to higher angles. This is attributed to the compressive stress caused by the difference in bonding properties of N and O 38 .The broad diffraction is attributed to the decrease in the grain size reduction with the destruction of crystal structure 39 . XRD spectra of the composites (NTM 1 , NTM 2 and NTM 3 ) with low and high dispersion of www.nature.com/scientificreports/ MoS 2 ; hence no MoS 2 diffraction peaks is seen in the spectra of NTMs composites 38 . Moreover, compared to the composites, MoS 2 shifts the (1 0 1) lattice plane peaks of anatase TiO 2 to higher angles, and the peak intensity decreases with increasing MoS 2 ratio. This is due to the large peak broadening, the peak became weaker and the affinity for amorphous structures was increased by the addition of MoS 2 40 . This indicates that MoS 2 is present in the NTMs composite (NTM 1 , NTM 2 and NTM 3 ). Figure 3 displays the FT-IR spectra of all the samples investigated, showing strong absorption bands in the range 400-700 cm −141 . This band is assigned to the stretching vibration of the Ti-O-Ti bond. This is related to the formation of TiO 2 and the observed shift in the composite spectra, indicating that the dopant is incorporated into the TiO 2 lattice. The peak near 1600 cm −1 is attributed to the aromatics.

Results and discussion
The C-C bond and the peak around 3400 cm −1 correspond to stretching vibration of OH bond. Another vibration band ranging from 1622 to 1796 cm −1 corresponding to the O-H bending mode was also observed. This may be attributed to the presence of H 2 O molecules adsorbed on TiO 2 42 . The FT-IR spectrum of NT shows several absorbance peaks compared to pure TiO 2 43 . For the MoS 2 spectrum, the bands at 608 cm −1 and 1058 cm −1 represent the characteristic peaks of MoS 2 . These characteristic peaks of MoS 2 are blue shifted in for NTM NCs. Therefore, the FT-IR results indicate the successful synthesis of TiO 2 , MoS 2 , and NTMs NCs 44 .
There are several mechanisms that control photocatalytic activity: electron/hole pair generation, light absorption, charge/carrier transfer, and carrier utilization. Optimization of photocatalytic activity depends on the efficiency of the products and the transfer of the e − /h + pairs, which depends on the energy band gap (Eg) of the photocatalyst. The energy band gap value (Eg) of the samples were determined using the following Eqs. 45,46 : where α is the absorption coefficient, v is the frequency of light and n is the constant of proportionality. The n value is determined by the transition of the semiconductor, i.e., the direct transition as in the prepared nanocomposite (n = 1). The diffuse reflection spectra (DRS) of T, NT, M and NTMs naocomposite were examined in the range of 200-800 nm as shown in Fig. 4. The pureTiO 2 NP has higher absorption band edge at around 422 nm compared with the NT displayed relatively steep in absorption band edge approximately 391 nm. In the presence of MoS 2 , the band gap of the MTN 1 , MTN 2 , and MTN 3 nanocomposites shifted toward the blue shift absorption edge around at 365 nm compared to pure TiO 2 . The addition of MoS 2 to the TiO 2 crystal lattice significantly increased the amount of visible light that was absorbed as a result of the effect of quantum confinement in MoS 2 with small band gap energy (1.23 eV) which corresponds to the long wavelength absorption edge (λ = 1040 nm) 47 . As a result, the absorption edge of the composites reached to a visible region. Table 1 shows the values of the optical band gaps of the samples have been estimated from the plots of reflection percentage versus energy (hʋ), there are two band gaps for all samples. The band gap (Eg) of the composites were between 1.5 and 2 eV and approaches to 3.2 eV for the T sample. It was shown that decreasing the bandgap energy of the composite enhances the photocatalytic process by absorbing more photons and increasing the photosensitivity The room temperature photoluminescence (PL) spectra of all prepared samples are shown in Fig. 5. The PL intensity in the fluorescence emission spectra of a semiconductor photocatalyst can be used to characterize the recombination of photogenerated electrons and photogenerated holes. The lower the PL intensity of photogenerated electrons, the more effective the separation of the photogenerated cavitation. The PL spectra of T and their nanocomposites, which were found similar. The peak at ~ 390 nm is consistent with the emission of anatase TiO 2 , and peaks around at 406, 420, 445 and 480 nm for NT, NTM 1 , NTM 2 and NTM 3 respectively. Among all the NTM catalysts, they exhibited lowest PL intensity, and obvious fluorescence quenching, indicating that the recombination of photogenerated electrons (e−) and holes (h+) is effectively suppressed. The PL results indicate that the two-dimensional MoS 2 layer with π-conjugated structure is an effective electron acceptor, and the   Fig. 6a. It can be clearly seen that the NTM composite materials has a higher MB photocatalytic activity degradation of than that of TiO 2 photocatalyst. Figure 6 measures the degradation at irradiation times of of 0, 30, 60, 90, 120 and 150 min. Before photocatalytic reaction, the photocatalyst's MB solution was kept for 30 min in the dark to reach the adsorption/ desorption equilibrium. This equation gives the efficiency of MB degradation: where C 0 is the initial concentration and C is the residual concentration of MB after the reaction. The efficiency of NTM 2 showed the highest photocatalytic degradation activity for MB dye, with a value of 99%, compared to pure TiO 2 (13%), NTM 1 (84.8%) and NTM 3 (80.8%). The incorporation of MoS 2 and N into TiO 2 lattice in appropriate amount led to a reduction in band gap energy and sufficient PL properties, and thus, the superior photocatalytic performance of the NTM 2 sample is directly related to it. According to the L-H kinetics model, the degradation kinetics of MB by the prepared nanocatalysts was evaluated. The pseudo-first-order kinetics equation can be expressed as: where k a is the rate constant (min −1 ), C 0 is the initial concentration (mg L −1 ), and C is the reaction concentration of the MB solution when the irradiation time is zero and t min. Figure 6b shows the relationship between ln (C 0 /C) and time. The rate constants (K a ) can be obtained from the linear relation between them are improved in the order shown in Table 1: NTM 2 > NTM 3 > NTM 1 > NT > T > MB. The NTM 2 photocatalyst has the largest rate constant (0.02178 min −1 ) compared to TiO 2 (0.0009 min −1 ) which is consistent with the photocatalytic  www.nature.com/scientificreports/ degradation results, showing that the catalyst has good characteristics and and good MB degradation activity under visible light. Therefore, the prepared MTN composites can act as effective photocatalysts to degrade organic compounds with good stability. In addition, as shown in Table 2, NTM 2 had the highest photocatalytic activity under visible light comparedto the results from previous studies. Reusability studies were examined by FT-IR after 150 min photocatalysis, as shown in Fig. 7a, the peaks perfectly correspond to the FT-IR peaks of the catalysis before the photocatalytic degradation reaction, there was no change in the peak position. These results show why, after many consecutive reuses, the prepared catalyst can maintain its catalytic efficiency as well as its stability. The degradation rate of the optimized photocatalyst NTM 2 for MB can be recycled as shown in Fig. 7b, and its photodegradation activity was found to decrease slightly after six cycles of use, indicating that its high stability. The stability property of NTM 2 can be attributed to the interaction between NT and MoS 2 , which can immobilize the active sites of NT nanoparticles in photocatalysis 48 . In addition to studying the roles of free radical as shown in Fig. 7c. We applied trapping agents of free radical: Tert-butyl alcohol (TBA), p-benzoquinone (BQ) and disodium ethylene di amine tetra acetic acid (Na 2 -EDTA) to scavenge the hydroxyl radicals, superoxide radicals and holes, respectively as present in Fig. 7c. The removal efficiency of MB changed dependent on the sacrificial agents, and the removal efficiency decreased to 45% in the presence of (5 mM) TBA as hydroxyl radicals played an important role in the photodegradation of MB. Figure 8 illustrates the degradation mechanism based on all previous results and the energy band of hereto structure of NTMs. When N-TiO 2 and MoS 2 are coupled together, photons can be absorbed on the surface of the photocatalyst, resulting in the formation of electron/hole pairs. The electrons from the conduction Band (CB) of N-TiO 2 will move to the band of MoS 2 , while holes from the valence band (VB) of N-TiO 2 will remain there. The possibility of electron/hole recombination is reduced by this procedure. As a result, compared to NT, the NTMs composite exhibited improved photocatalytic activity. The OH − in the aqueous solution is then absorbed across the hole in the valence band to give a highly reactive OH . Radical. Finally, these active • O 2 radicals, h + , and • OH radicals interact with MB molecules adsorbed on the surface of NTM NCs photocatalyst molecules and degrade    www.nature.com/scientificreports/

Conclusion
In summary, for more efficient MB degradation from wastewater, a nitrogen-doped titania-molybdenum sulfide (NTM) nanocomposite was used to photodegrade MB under visible light irradiation. The photocatalytic activity of NTM NCs was investigated by the degradation of methylene blue dye under visible light irradiation. It is observed that the NTM 2 photocatalyst has a stronger visible light absorption and is seven times higher than that of pure TiO 2 . The heterostructure formation between MoS 2 and NT, possessing the synergistic effects of enhanced MB adsorption on the catalyst surface, better visible light absorption, efficient charge transport and separation, which is highly responsible for the enhanced photodegradation activity of MB on NTMs catalyst under visible light irradiation. Reusability experiments show very high stability of NTMs. Therefore, the prepared catalyst is an excellent candidate for the efficient photocatalysis of toxic pollutants from aqueous solution.