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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.6 pp.353-366
DOI : https://doi.org/10.3740/MRSK.2021.31.6.353

Photocatalytic Generated Oxygen Species Properties by Fullerene Modified Two-Dimensional MoS2 and Degradation of Ammonia Under Visible Light

Cong-Yang Zou1, Ze-Da Meng1, Wei Zhao2, Won-Chun Oh3
1Suzhou University of Science and Technology, Jiangsu Key Laboratory of Environmental Functional, Suzhou 215009, China
2Dept. of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, P. R. China
3Department of Advanved Materials Science and Engineering, Hanseo University, Seosan-si, Chungnam-do 356-706, Republic of Korea
Corresponding author E-Mail : compelitely@163.com (Z.-D. Meng, Suzhou Univ. Sci. Tech.) wc_oh@hanseo.ac.kr (W.-C. Oh, Hanseo Univ.)
December 2, 2020 May 22, 2021 June 3, 2021

Abstract


In this study, photocatalytic degradation of ammonia in petrochemical wastewater is investigated by solar light photocatalysis. Two-dimensional ultra-thin atomic layer structured MoS2 are synthesized via a simple hydrothermal method. We examine all prepared samples by means of physical techniques, such as SEM-EDX, HRTEM, FT-IR, BET, XRD, XPS, DRS and PL. And, we use fullerene modified MoS2 nanosheets to enhance the activity of photochemically generated oxygen (PGO) species. Surface area and pore volumes of the MoS2-fullerene samples significantly increase due to the existence of MoS2. And, PGO oxidation of MB, TBA and TMST, causing its concentration in aqueous solution to decrease, is confirmed by the results of PL. The generation of reactive oxygen species is detected through the oxidation reaction from 1,5-diphenyl carbazide (DPCI) to 1,5-diphenyl carbazone (DPCO). It is found that the photocurrent density and the PGO effect increase in the case with modified fullerene. The experimental results show that this heterogeneous catalyst has a degradation of 88.43% achieved through visible light irradiation. The product for the degradation of NH3 is identified as N2, but not NO2−or NO3−.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Pouring chemical wastes from industrial plants and manufactures to the environment is an important factor in water pollution which can eventually pollute the water sources.1) These wastes include solvents and toxic substances. Ammonia is a common water contaminant that has significant effects on the environment and human health. The ammonia is one of the important nitrogen-containing pollutants and a source of nutrients that may accelerate the eutrophication and cause algal growth in natural water.

    Different technical methods were applied to remove ammonia from industrial wastewater, such as biological treatments, chemical precipitation, advanced oxidation processes, air stripping, ion exchange, adsorption, membrane, and photocatalytic processes. The photocatalytic degradation of various toxic compounds has been proposed as a practical method in the decontamination of wastewater for renewable use.2)

    Molybdenum disulfide MoS2 has a layered structure like graphite and because of its unique properties, has been known and used in chemical reactions for several years.3) Among these properties, anisotropy, chemically inertness, photo-corrosion resistance and specific optical properties are included as the superior properties. As a catalyst, MoS2 carry the advantage of being reactive.4) Unlike graphite and layered hexagonal BN (h-BN), the monolayer of MoS2 is composed of three atom layers: a Mo layer sandwiched between two S layers.5) The triple layers are stacked and held together through weak van der Waals interactions. Most recently, it is proposed that the indirect band gap of bulk MoS2 with a magnitude of approximately 1.2 eV transforms gradually to a direct band gap of approximately 1.8 eV in single-layer samples,6,7) which is in contrast to pristine graphene with a band gap of about 0 eV and few-layered h-BN with a band gap of about 5.5 eV.8,9) All these results imply that 2D MoS2nanosheets have potential applications in electronics, optics, and semiconductor technologies as promising complements to graphene and h-BN.10)

    Many studies have shown that heterogeneous photocatalysis through light illumination on a semiconductor surface is an attractive advanced oxidation process.11-14) Under irradiation, the semiconductor is excited to generate hole and electron pairs, and the holes subsequently react with adsorbed hydroxyl ions to generate hydroxyl radicals, while the electrons react with adsorbed oxygen to generate superoxide ions.11) These oxidizing substances, which have higher reaction activity than common oxygen molecules, can completely destroy various organic pollutants in wastewaters. In order to enhance the efficiencyof photochemically generated oxygen (PGO) species and to search for new highly active photocatalysts, it is necessary to study new catalysts.

    Unfortunately, deposited metal particles or coupled with other semiconductors only serves as electron trapping agents or causes the transfer of photogenerated electrons, and is not effective in enhancing the adsorption of the pollutants. Fullerene (C60) treated TiO2coupling with other semiconductors has been reported to perform both the above-mentioned functions.12) Additionally, C60 is one of the most promising materials for this application because of its band gap energy, being about 1.6 ~ 1.9 eV. It has strong absorption in the ultraviolet region and weak but significant bands in the visible region. In general, coupled systems exhibit high degradation rates as well as an increased extent of degradation different dyes.13,14)

    1,5-diphenyl carbazide (DPCI) can be oxidized by oxidizing substances into 1,5-diphenyl carbazone (DPCO), which can be extracted by organic solvents and display an obvious absorbance in a certain range of wavelength.15)

    In this work, a new highly active photocatalyst was prepared to solve the increasingly serious environmental problems. We synthesized the MoS2 coupled with different content of fullerene, which combines the excellent charge transport property of the MoS2 and the absorption property of fullerene. These new photocatalysts exhibited enhanced vis-photocatalytic activity. These catalysts were irradiated with visible light and their catalytic activities were compared. The objective of this paper is to experimentally prove that the ability of the photochemically generated oxygen species of fullerene-decorated MoS2 composites is superlative in visible light using MB, TBA and TMST and 1,5-diphenyl carbazide (DPCI).

    2. Experimental

    2.1 Materials

    Benzene (99.5 %) and ethyl alcohol were purchased as reagent-grade from Duksan Pure Chemical Co. (Korea) and Daejung Chemical Co. (Korea), and used as received. Crystalline fullerene [C60] powder (99.9 % purity from Tokyo Kasei Kogyo Co. Ltd., Japan) was used as the carbon matrix. Sodium thiosulfate pentanhydrate (Na2S2O3·5H2O) and sodiummolybdatedihydrate (Na2MoO4·2H2O) were supplied by Duksan Pure Chemical Co., Ltd, Korea.

    2.2 Oxidation of fullerene surface

    MCPBA (m-chloroperbenzoic acid, ca. 1 g) was suspended in 50 mL of benzene, followed by the addition of fullerene [C60] (ca. 40 mg). The mixture was then refluxed in an air atmosphere and stirred for 6 h. The solvent was subsequently dried at the boiling point of benzene (353.13 K). After completion, the dark brown precipitates were washed with ethyl alcohol and dried at 323 K, after which the oxidized C60 was formed.

    2.3 Synthesis of MoS2-fullerene composite

    The oxidation fullerenes nanomaterials (30 mg, 60 mg and 100 mg) were added to Na2MoO4·2H2O and Na2S2O3·5H2O mixture solutions, respectively. After stirring and ultrasonication the mixture was added into a 100 mL Teflon-line autoclave and blent ethylene diamine. Then the Teflon-line autoclave maintained at 453 K for 36 h. The solvent was evaporated and the MoS2-fullerene powders were obtained after being dried (at 50 °C).

    2.4 Characterization of MoS2-fullerene compounds

    For the measurement of structural variations, XRD patterns were taken using an X-ray generator (Shimadzu XD-D1, Japan) with Cu Ká radiation. SEM was used to observe the surface state and structure of composites using a scanning electron microscope (JSM-5200 JOEL, Japan). Energy dispersive X-ray (EDX) spectroscopy was also used for the elemental analysis of the samples. The N2 adsorption isotherm was measured at 77 K using a BEL sorp Analyzer (BEL, Japan). Then, the BET surface area was calculated by nitrogen adsorption. The pore size distribution was calculated by the BJH method.16) Highresolution transmission electron microscopy (HRTEM, JEM-3010, Japan) was used to observe the surface state and structure of the MoS2-fullerene composites. At the acceleration voltage of 200 kV, HRTEM was used to investigate the size and distribution of the titanium and iron particles deposited on the fullerene surfaces of various samples. HRTEM specimens were prepared by placing a few drops of the sample solution on a carbon grid. The absorbance spectrum of the photocatalyst was recorded at room temperature in the light range 300-800 nm using a UV-Vis spectrophotometer (Genspec III, Hitachi, Japan) equipped with an integration sphere.

    2.5 Catalytic degradation of MB, TBA and TMST (telmisartan)

    The photocatalytic activities were evaluated by MB degradation in aqueous media under visible light irradiation. For visible light irradiation, the reaction beaker was oriented axially and held in a visible light box (8 W, halogen lamp, KLD-08L/P/N, Korea). The luminous efficacy of the lamp is 80 lm/W, and the wavelength is 400 nm ~ 790 nm. The lamp was used at a distance of 100 mm from the aqueous solution in a dark box. The initial concentration of the MB was set at 2 × 10−5 mol/L in all experiments. The amount of the photocatalyst composite was 0.05 g/(50 mL solution). On the other hand, 0.001 v/v aqueous solution of TBA and 0.0025 v/v aqueous solution of TMST were also prepared with deionized water in 1 L measuring flasks, respectively.

    The reactor was placed for 30 min in a darkness box, in order to make the photocatalyst composite particles adsorb the maximum possible amount of MB, TBA and TMST molecules. After the adsorption state was reached, the visible light irradiation was restarted to make the degradation reaction proceed. In the process of the degradation of methylene blue, a glass reactor (diameter = 4 cm, height = 6 cm) was used and the reactor was placed on a magnetic churn dasher. The suspension was then irradiated with visible light for a set irradiation time. Visible light irradiation of the reactor was done for 30 min, 60 min, 90 min and 120 min, respectively. Samples were withdrawn regularly from the reactor and dispersed powders were removed by using a centrifuge (1 min, 1000 r/min). The clean transparent solution was analyzed by UV-vis spectroscopy. In case of MB the spectral range investigated was from 500 nm to 750 nm whereas in case of TBA the spectral range was from 250 nm to 350 nm and in case of TMST from 370 nm to 400 nm. The absorbance value at the λmax of the dye solutions was obtained. The MB, TBA and TMST concentration in the solution was determined as a function of the irradiation time.

    2.6 Evaluation of reactive oxygen species

    Firstly, six 10.00 mL DPCI stock solutions (1.00 × 10−2 mol/L) were added into three 80 mL volumetric flasks, respectively. And then 50 mg three different MoS2- fullerene photocatalysts were added to above DPCI solutions, respectively. All of the three solutions were diluted to 50 mL with double distilled water. For all three solutions, the final DPCI concentration and MoS2- fullerenephotocatalyst amount were 2.00 × 10−3 mol/L and 1.00 g/L, respectively. Among them, the reactors were put into a visible light apparatus away from light directly under visible light irradiation. After 180 min irradiation, from each sample 10.00 mL solution was taken exactly and extracted with benzene. And then, all extracted solutions were diluted to 10.00 mL with benzene solution and their UV-vis spectra were determined.

    2.7 Degradation of NH3

    Photocatalytic experiments for NH3degradation were conducted under visible light irradiation (λ > 400 nm). A 300W UV–visible lamp (OSRAM, Germany) was used as a light source. Photo-Fenton degradation of 50.0 mL of NH3 solution was performed in a 100 mL beaker at room temperature (25 ± 2 °C). The distance between the lamp and test solution was approximately 10cm, and the wall of the beaker was shielded from surrounding light by aluminum foil. Visible light was allowed to pass through aλ>400 nm cut-off filter covering the window of the beaker; this filter absorbed UV light and allowed visible light of λ>400 nm to pass through. In a typical photo-Fenton experiment, 50 mL of test solution was used. The NH3solutions were prepared according to the desired concentrations, and 0.5 g of the MoS2-fullerene catalyst was used for the photocatalytic experiments.

    A double-beam TU-1901 spectrophotometer was used to determine the concentration of NH3by reaction with Nessler reagent during the photo-Fenton process. Nessler reagent is an alkaline solution of dipotassium tetraiodomercurate (II), this reagent was prepared by dissolving 10 g of HgI2 and 7 g of KI in water, adding to NaOH solution (16 g NaOH in 50 mL of water), and then diluting with deionized water to 100 mL. The reagent was stored in dark bottles and diluted properly before analysis. NH3reacts with this reagent to yield colored solutions via Reaction (1). As the absorbance of the solutions showed a maximum value at 392 nm, absorbance was measured at a wavelength of 392 nm:

    NH 4 + + 2[HgI4] 2- + 4HO - HgO Hg(NH 2 )I + 7I - + 3H 2 O
    (1)

    [HgI4]2− is yellow and HgO·Hg(NH2)I is brown.

    3. Results and Discussion

    3.1 Elemental analysis of the samples

    Fig. 1 shows the EDX patterns of the MoS2-fullerene composites. As shown in Fig. 1, in most samples, C, S and Mo were present as major elements with small quantities of oxygen in the composite. There were also some small impurities, which were attributed to the use of fullerene without purification. Peaks appeared at 2.2 keV are due to Mo.17) The S peak is overlapping with Kα peak of Mo and is not resolvable in the figure. Moreover, the numerical results of EDX quantitative microanalysis of the samples are listed in Table 1. The ratio of Mo/S was detected as 1/1.98 which is in complete agreement with the chemical formula of MoS2 showing that no any significant residue of the starting materials or the surfactant is remained in the product. In addition, the C content shows an increasing tendency when increasing the content of fullerene.

    3.2 Surface characteristics of the samples

    The micro-surface structures and morphologies of the three differentMoS2-fullerene composites were characterized by SEM (Fig. 2). As shown in Fig. 2(a), the SEM image for pure MoS2, shows the laminated structure of MoS2 clusters, which can also be found in the MoS2-fullerene composites. MoS2 has a small particle size and a good dispersion and the fullerene particles are shown as spherical particles with small facets and have a good dispersion as well.18) Particle sizes around 150 ~ 200 nm can be estimated from the micrograph for the hydrothermal method while they are smaller (around 100 nm) and more uniform in size and shape for particles synthesized by the modified hydrothermal method. It is clear, from Fig. 2(b-d), that the particles are too much bigger and they differ in size and shape extensively. Therefore, it can be concluded that sample added fullerene created particles to become larger during the modified hydrothermal method. We conjecture that the spherical particles in MoS2-fullerene composites are fullerene clusters or fullerene particles on the MoS2 surface. A good dispersion of small particles could provide more reactive sites for the reactants than aggregated particles.19) At the same time, the conductivity of fullerene can facilitate electron transfer between the adsorbed dye molecules and the catalyst substrate. By comparing Fig. 2(b), (c) and (d), with increasing content of fullerene, the dispersion of fullerene has not deteriorated. We also find the amount of fullerene particles was increased when increasing the content of fullerene.

    Fig. 3 shows HRTEM images of the MoS2-fullerene composites. HRTEM is a technique used for analyzing the morphology, crystallographic structure, and even the composition of a specimen. Fig. 3 gives direct evidence that the fullerenes are well contacted with MoS2. In Fig. 3(a), ribbon like stacking of S-Mo-S layers are detectable inside the MoS2 particles. These are completely disordered especially at the corner. However, accumulations are seen in the core regions of the particles. This describes the morphology mostly known as “rag” morphology. The ribbons have thicknesses of approximately 2-4 nm and width of around 200 nm. The observed morphology is in complete agreement with results obtained from XRD studies which will be discussed in later section. The HRTEM image of the particles obtained by the modified procedure is shown in Fig. 3(b). The rag morphology is observed again in the morphology for the fullerene modified MoS2 particles. This indicates that the surfaces of the MoS2nanosheets are cleaned under exposure to the reaction conditions. Fig. 3 shows large clusters with an irregular agglomerated dispersion of fullerene. Fullerene particles were distributed outside the surfaces of the MoS2nanosheets with a size of approximately 1 nm, even though this caused partial agglomeration to form block particles. HRTEM images also revealed the presence of fullerene particles on the MoS2 nanosheets. It is revealed that the hexagonal lattice structure has the lattice spacing of 0.27 and 0.16 nm assigned to the (100) and (110) planes, respectively.20,21)

    Fig. 4 shows the FT-IR spectra of oxidized fullerene, MoS2 and MoS2–fullerene. The FT-IR spectrum of fullerene was rather simple and suggested extensive oxidation. The spectrum of the crystalline material showed well distinguished and sharp bands, whereas the amorphous spectra were less resolved. The hydration results established the importance of defined conditions for FT-IR (Fig. 4) and showed that fullerene could be oxidized. They also suggested that this type of study can be performed on oxidized fullerene. The FT-IR spectrum of MoS2– fullerene was similar to that of oxidized fullerene.

    From Fig. 4(a), the peak at 671 cm−1 was assigned to the alkane bending vibration, which occurs between 650- 1,000 cm−1, and the peaks at 705, 752 and 807 cm−1 were assigned to aromatic symmetric stretching modes, which occur in the range 690 ~ 900 cm−1. Strong C-O bands at approximately 1,253 and 1,288 cm−1, and a strong C=C band at 1,475 cm−1 were observed. The functional groups, C=O and C-OH, were indicated in the spectrum at approximately 1,693, 1,797 and 1,072 cm−1, respectively. Meanwhile, -OH was observed at approximately 1,407 cm−1.

    This confirms that artificial ageing is actually occurred on the surface, and that the types of structural changes inferred from the spectra are consistent with the mechanism proposed in the literature: the formation of O-H bonds resulting from the oxidation of the hydrocarbon triterperpenic molecules by the direct binding of O· and O-O radicals, followed by further oxidation to carbonylic functional groups.

    In Fig. 4(b), the results indicate that there is only one weak absorption peak at 474.1 cm−1 for the pristine MoS2 powder, which can be ascribed to characteristic Mo-S stretching vibration mode.22) Note that the MoS2 nanosheets have the same FTIR result as the pristine MoS2 powder, indicating that there is no absorbed element induced during the experiment progress. We also found this peak from Fig. 4(c).

    In Fig. 4(c), the weak peaks at 1,092 cm−1, 1,259 cm−1 and 1,294 cm−1 were assigned to the C-OH and C-O bands, respectively. The C=C functional groups were observed at approximately 1,463 and 1,693 cm−1, respectively, while C=O was indicated in the spectrum at approximately 1,787 cm−1. Compared to the spectrum curves (a) and (c), the peak intensity of the functional group on oxidized fullerene was weak and decreased. This is because some of the functional groups had combined with MoS2 particles. The MoS2 particles were bound to fullerene with different functional groups.

    3.3 Physico-chemical and surface properties

    The MoS2–fullerene composite catalysts derived from MoS2 and different contents of fullerenes were denoted as MoS2-C1, MoS2-C2 and MoS2-C3. As shown in Fig. 5, all materials also show type IV isotherms that are typical characteristics of mesoporous materials. The N2 adsorption–desorption isotherms of the hybrid display type IV isotherms with a distinct hysteresis loop of H3, suggesting the mesoporous and sheet structure of the MoS2. This observation further demonstrates that the incorporation of fullerene does not destroy the mesoporous structure. Type IV isotherm are its hysteresis loop, which is associated with capillary condensation taking place in mesopores, and the limiting uptake over a range of high p/po. The initial part of the Type IV isotherm is attributed to monolayer-multilayer adsorption since it follows the same path as the corresponding part of a Type II isotherm obtained with the given adsorptive on the same surface area of the adsorbent in a non-porous form. Table 2 summarizes the element content, BET surface areas, pore volumes and pore diameter of different samples. Generally, the specific surface area increases after fullerene being modified. The specific surface area and the pore volume of the samples increase with increased fullerene loading.23) Thus the sample MoS2-C3 has a higher BET surface area than other samples. While the pore diameter of MoS2demonstrated the presence of a large number of pores in the broad pore distribution ranges of 2.1-4.2 nm, the pore diameter of MoS2–fullerene composite is in the range of 10-40 nm.

    3.4 Structural analysis

    The X-ray diffraction patterns, shown in Fig. 6, are for MoS2 in their most stable hexagonal crystalline structure.24) The most important XRD feature which provides a proof for existence of the hexagonal unit cells is the observation of diffraction peaks due to (002) planes. This is the most prominent peak in Fig. 6 while diffractions from (100), (103), and (110) planes are appeared in smaller intensities. The X-ray diffraction pattern for MoS2 prepared by the hydrothermal method is shown in Fig. 6(b). Peak (002) is appeared in its characteristic region 2θ = 14.5 as a broad peak. Other detectable peaks are (100), (103) and (110). This combination of the peaks reveals the hexagonal structure for the prepared particles and the broadening of the peaks is in complete agreement with morphological studies and results obtained by TEM. Therefore, in comparison with other samples [Fig. 6(b-d)], a semicrystalline structure is assigned for the fullerene modified MoS2 particles. X-ray diffraction pattern for MoS2 nanoparticles prepared by the fullerene modified hydrothermal method is shown in Fig. 6(b-d). The peak (002) around 2θ = 14.5 is appeared and some peaks due to diffractions from (100) and (110) planes are detectable in smaller intensities. In comparison with Fig. 6(a), the peaks are broader to some extent, i.e. less crystallinity. This is consistent with the results obtained by TEM. In general, the fullerene modified hydrothermal method is accompanied with particles which are smaller and less crystalline compared to products obtained by the hydrothermal method.

    The bonding characteristics and the composition of the exfoliated MoS2 samples were captured by XPS. Results indicate that the wide XPS spectra of the exfoliated MoS2 sample show only signals arising from elements Mo and S. The Mo 3d XPS spectrum of MoS2 nanosheets, reported in Fig. 7(b), shows two strong peaks at 229.3 and 232.5 eV, which are attributed to the doublet Mo 3d5/2 and Mo 3d3/2, respectively,while the peak at 225.6 eV can be indexed as S 2s.25) The peaks, corresponding to the S 2p1/2 and S 2p3/2 orbital of divalent sulfide ions (S2−), are observed at 163.3 and 162.1 eV [shown in Fig. 1(c)], respectively. All these results are consistent with the reported values for the MoS2 crystal.26)

    Fig. 7 shows the XPS C(1s) and O(1s) spectra of the MoS2-fullerene sample. The C(1s) peak can be clearly deconvoluted into two subpeaks, namely, the CI peak centered at 284.7 eV and CII peak at 287.2 eV. The CI peak can be attributed to non-oxygenated and non-nitrogenated carbons of the C60 cage based on prior references.27,28) The CII peak can be assigned to the oxygenated C60 based on the fact for C-OH group.The O(1s) signal can also be clearly deconvoluted into two subpeaks, namely, the OI peak at 531.7 eV and the OII peak at 532.7 eV. The OI peak is most likely associated with a hydroxide species29) connected directly with carbon atoms, whereas the OII peak can be definitely assigned to the oxygen atoms for covalent C-O bonds. This observation further demonstrates that the fullerene was oxidized and has functional groups on the surface.

    3.5 UV-vis diffuse reflectance spectroscopy

    The UV-vis absorption spectra of MoS2-fullerene samples are shown in Fig. 8. We find that all of the composites have great absorption in the ultraviolet and visible regions, from which we can calculate that these composites have great photocatalytic activity under ultraviolet light and visible light irradiation. Due to MoS2 having a relatively small band gap, MoS2 has photocatalytic activity in the visible region. For the MoS2nanosheets, peak positions for A and B excitons are at 702 nm (1.77 eV) and 644 nm (1.92 eV), respectively.30) We can find these two peaks from the inset image.

    When fullerene was coupled with MoS2, the fullerene acted as an energy sensitizer which improved the quantum efficiency and increased the level of charge transfer. Because of the synergistic reaction of fullerene and MoS2, the adsorption effect of MoS2–fullerene is good in both the ultraviolet and visible regions. By comparing the UV-vis absorption spectra of MoS2 and MoS2–fullerene, we can see that the adsorption effect of MoS2–fullerene is better than that of pure MoS2.

    3.6 Photoluminescence spectra of samples

    Photoluminescence (PL) was used to study the excited state and reveal the differences in the photocatalytic performances of samples. It can be seen that, the PL spectra of the hierarchical MoS2-fullerene composites are similar to that of pure MoS2. The two peaks in the absorption spectrum, at 619 and 669 nm, correspond to B and A excitons, respectively. The emission peaks of A and B excitons are characteristic exciton peaks of MoS2 monolayer whose energy separation is due to spin-orbit splitting at the top of the valence band at the K point of the 1st Brillouin zone.31) However, an obvious fluorescence decreases (or quenching) of the hierarchical MoS2- fullerene composites can be observed compared with that of MoS2, indicating a much lower recombination rate of photoinduced electrons and holes. The PL results demonstrated that theMoS2layers with a two-dimensionalconjugated structure couldserve as an effective electronaccepting material and the formed strong interaction between fullerene and MoS2could improve the separation of electron–hole pairs under visible light irradiationand then electrons easily transfer to MoS2sheets, inhibiting a direct recombination of electrons and holes.32)

    3.7 Photocatalytic activity of samples

    3.7.1 Degradation of MB, TBA, TMST and DPCI

    Fig. 10 shows the time series of MB degradation using pure MoS2 and three different MoS2–fullerene composites under visible light with different irradiation intensities. The spectra for the MB solution after visible light irradiation show relative degradation yields at different irradiation times. The dye concentration continuously decreased with an excellent rate, which was due to visible light irradiation. The concentration of MB was 2.0 × 10−5 mol·l−1, and the absorbance for MB decreased with increasing visible light irradiation time. Moreover, the MB solution increasingly lost its color as the MB concentration continued to decrease. Two steps are involved in the photocatalytic decomposition of dyes: the adsorption of dye molecules and degradation. After adsorption in the dark for 30 min with magnetic stirring, the samples reached adsorption–desorption equilibrium. In the adsorptive step, MoS2 and MoS2–fullerene composites showed different adsorptive effects, with MoS2–fullerene having the best adsorptive effect. The adsorptive effect of MoS2–fullerene was better than that of MoS2 because the added fullerene can enhance the BET surface area which can increase the adsorption effect. MoS2-C3 has the largest BET surface area, which enhances the adsorptive effect. The MoS2– fullerene composites showed good adsorption and degradation effects. A comparison of the decoloration effects of the different catalysts showed that the degradation effect can be increased by an increase in the adsorption capacity.

    Fig 11(a), (b) represent the degradation of TBA and TMST with MoS2-C3 in visible light from which it is clear that the concentration of TBA and TMST gradually diminishes with increasing time for all of samples. The decreasing concentration of TBA and TMST in the photocatalytic reaction was used to evaluate the activity MoS2-C3 composites. The spectra of the dye solution show the relative degradation yields at different time intervals. Moreover, the dye solution increasingly lost its colour as the dye concentration continued to decrease. The decrease in concentration was evaluated at the λmax values of the dyes which were determined from the absorption spectra of the dyes. The λmax values of MB, TBA and TMST were found to be 665 nm, 265 nm and 375 nm, respectively. However, due to confidential issues of the manufacturing company, information about the textile dyes TBA and TMST the detailed reference were not found. But the UV/Vis spectroscopic analysis and the rate determination of the drastic diminish in the dye concentration clearly indicated photocatalysis.

    Fig. 11(c) is the UV-vis spectra of OPCO extract liquors in the presence of MoS2-C3 composite under visible light irradiation. 1,5-diphenyl carbazide (DPCI) can be oxidized by oxidizing substances into 1,5-diphenyl carbazone (DPCO). Under visible light irradiation, the MoS2-C3 samples turns into the excited state. That is, some electrons are transited from valence band (VB) to conduction band (CB). Simultaneity, the electron–hole pairs form on the surface or in the inner of MoS2-C3 samples. The electrons and holes react with the molecular oxygen (O2) dissolved in aqueous solution and water molecules (H2O) absorbed on the surface of MoS2-C3 particles, producing the superoxygen radical anions ( · O 2 ) and hydroxyl radicals (•OH), respectively. These •OH can oxidize 1,5-diphenyl carbazide (DPCI) into 1,5-diphenyl carbazone (DPCO). The DPCO can be extracted by the solvent of benzene and show an absorbance at 560 nm wavelength. Sequentially, the produce and output of •OH can be easily detected, and the photodegradation activity of MoS2-fullerene can be easily detected. From Fig. 10 we can see that, under visible light irradiation, the absorption peaks of DPCO around 560 nm show an obvious increase compared with the corresponding ones without any irradiation.33) And that, for different irradiation time under visible light the DPCO solution exhibits different absorbance.

    3.7.2 Degradation of NH3

    The degradation curves of NH3-N with MoS2-C3or MoS2 as the catalyst in the presence or absence of H2O2 under visible light irradiation or in the dark are presented in Fig. 12. An abatement ratio of 10.90 % may be observed in Curve 1 after reaction for 10.0 h in the dark. This result shows that the percentage of NH3 adsorbed on MoS2-C3 catalyst is 10.90 % (the adsorbed of NH3 by MoS2 catalyst is 8.51 % under similar conditions). Curve 2 shows an NH3-N abatement ratio of 21.60 %; this result indicates that H2O2 enables slight degradation of NH3 in the dark. Visible light irradiation of the solution represented by Curve 1 caused 29.78 % NH3-N abatement at 10.0 h (Curve 4) in the absence of H2O2, and in the absence of H2O2 and MoS2-C3 catalyst the NH3-N abatement ratio at 10.0 h was 21.50 % (Curve 3), thereby confirming that the MoS2-C3 catalyst cannot thorough degrade NH3 via photogenerated holes except the adsorption. Visible light irradiation of solutions containing 1.0 mmolL−1 H2O2 without catalysts showed a little degradation of NH3 (Curve 5). The MoS2 catalyst achieved 55.70 % NH3 degradation (Curve 6). The presence of MoS2-C3 hybrid catalyst in the test solution achieved 88.43 % NH3 degradation (Curve 7); such a result demonstrates that the MoS2-C3 hybrid catalyst is an effective photo-Fenton catalyst for NH3 degradation.

    3.7.3 Effect of H2O2 concentration

    To examine the effect of H2O2 concentration on the degradation ratio, a series of solutions containing 0.10, 1.0, 5.00, or 10.0 mmol/L H2O2 were tested under visible light irradiation. The results showed that the degradation ratios of NH3 in these solutions approached 62.0 %, 77.6%, 78.7 %, and 81.3%, respectively, after 180 min of reaction. As shown in Fig. 13, the degradation ratio of NH3 rose with increasing H2O2 concentration and approached aplateau at H2O2 concentrations over 1.0 mmol/L. Compared with the solution with 5.0 mmol/L H2O2, the solution with 10.0 mmol/L H2O2 caused only 2.6 % increase in NH3 degradation ratio. Therefore, 1.0 mmol/L H2O2 was utilized in subsequent experiments.

    3.8 Oxidation mechanism of NH3

    NH3 could be oxidized to form final products of NO 2 - , NO 3 - , and N2 via different reaction pathways. The absorbance peaks of NO 3 and NO 2 anions may be observed at 206 and 211 nm,34) respectively. Therefore, absorbance detected at 206 and 211 nm reveals the formation of NO 3 and NO 2 anions. Consecutive measurements did not detect any absorbance in the wavelength range of 200 ~ 230 nm during the photo-Fenton process, thereby confirming that the photocatalytic process does not yield nitrate or nitrite.

    NH3 oxidization occurs through two pathways. The first pathway forms N2 as the final product through a series of NH2, NH, and N2H(x+y:x,y=0,1) intermediates.35) The second pathway yields NO 2 and NO 3 ions as final products through a HONH2 intermediate. Because NO 2 and NO 3 were not detected in this work, as previously described, and the final product NH3oxidization in the reaction system is N2,36) the relevant reaction mechanism could be described as in Eqs. [(2), (3), (4), (5) and (6)]:

    NH 3 + · OH NH 2 + H 2 O
    (2)

    NH 2 + · OH NH + H 2 O
    (3)

    NH + · OH N + H 2 O
    (4)

    NH x + NH y N 2 H ( x+y:x,y=0,1 )
    (5)

    N 2 H ( x+y ) + ( x + y ) OH N 2 + ( x+y ) H 2 O
    (6)

    The MoS2–fullerene hybrid catalyst can photocatalytically oxidize NH3 to N2, thereby achieving thorough removal of NH3. This reaction mechanism is presented in Fig. 14.

    MoS2–fullerene has a better degradation effect than pure MoS2 because fullerene is an energy sensitizer which improves the quantum efficiency and increases the level of charge transfer. When C60 is irradiated with UV-vis radiation, it is excited from the ground state to a short lived singlet excited state (ca. 1.2 ns), which undergoes rapid intersystem crossing at a rate of 5.0 × 108/s to a lower lying triplet state ( 3 C 60 * ) with a long lifetime (> 40 μs). More importantly, photo-excited fullerenes are also excellent electron acceptors capable of accepting as many as six electrons. 3 C 60 * has a higher electron accepting ability than ground state 1C60, and electron-donating compounds can reduce 3 C 60 * to give the C60 radical anion ( 3 C 60 ). As the potential of the CB of MoS2 is equal to 0.23 eV and the potential of the transformation ( 3 C 60 * / 3 C 60 · ) is equal to -0.2 eV, electrons may be transmitted from the conduction band of MoS2 into the deposited 3 C 60 * , resulting in the formation of 3 C 60 · . This radical species may react further with reactants adsorbed at the interface.37,38) Thus, the deposition of C60 on the MoS2 surface is expected to improve the photocatalytic behavior. Otherwise, fullerene can generate e/h+ pairs under visible light irradiation. It can be proposed that e transfer also happens in the MoS2-fullerene composites between fullerene and MoS2, retarding the e/h+ recombination and increasing the photon efficiency. The light absorption capability of the photocatalyst and the separation of photo-generated e/h+ pairs are crucial factors influencing the photo-activity.39,40)

    MoS 2 + hv e - + h +
    (7)

    C 60 + hv 3 C 60 *
    (8)

    3 C 60 * + e · 3 C 60 ·
    (9)

    C 60 + hv e - + h +
    (10)

    h + + OH · HO
    (11)

    e - + ( O 2 ads ) O 2 ·
    (12)

    Based on the above experimental results, we can conclude that this degradation mechanism is suitable to explain the results of MB photocatalytic degradation using the prepared MoS2-fullerene composite. Fig. 14 shows a schematic diagram of the separation of photogenerated electrons and holes on the MoS2-fullerene interface and the photocatalytically oxidize NH3 to N2.

    4. Conclusion

    In this study, two-dimensional (2D) MoS2 and the PGO properties of MoS2-fullerene composites were investigated and exhibited enhanced photocatalytic activity. The MoS2- fullerene catalyst can degrade NH3 effectively into nitrogen gas under visible light irradiation. The FT-IR results illuminated that the MoS2 band transfers electrons to fullerene with functional groups attached. The adsorption and surface properties as structural and chemical composition of the MoS2-fullerene composites were investigated. Surface areas and pore volumes of the samples were increase due to deposition of fullerene. The XRD and XPS results illuminated that the phase type was hexagonal MoS2 phase and fullerene. UV-vis patterns indicated that both MoS2 and MoS2-fullerene have good photo-induction effects in the visible light region. The quantities of generated hydroxyl radicals can be analysis by DPCI degradation. The results demonstrated that the PGO oxidation of MB, TBA and TMST causing its concentration in the aqueous solution to decrease, which is confirmed by the results of PL. The MoS2-fullerene composite catalyst prepared by the impregnation method demonstrates higher PGO activity than pure MoS2. MoS2-C3 has the best degradation and adsorption effect due to the high photo sensitivity and relativity high adsorption effect of fullerene. The catalyst proposed in this work is expected be applicable in degradation of recalcitrant NH3.

    Acknowledgments

    The work was supported by the National Natural Science Foundation of China (Nos. 51502187), the JiangSu Collaborative Innovation Center of Technology and Material for Water Treatment, and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

    Figure

    MRSK-31-6-353_F1.gif

    EDX elemental microanalysis of samples: MoS2 (a) MoS2- C1 (b), MoS2-C2 (c) and MoS2-C3 (d).

    MRSK-31-6-353_F2.gif

    SEM images of samples: MoS2 (a), MoS2-C1 (b), MoS2-C2 (c), and MoS2-C3 (d).

    MRSK-31-6-353_F3.gif

    HRTEM image of the samples: MoS2 (a), MoS2-fullerene (b).

    MRSK-31-6-353_F4.gif

    FT-IR spectra of samples: oxidized fullerene (a), MoS2 (b), MoS2-fullerene (c).

    MRSK-31-6-353_F5.gif

    Nitrogen adsorption/desorption isotherms of samples, inset image is the pore-size distribution for samples.

    MRSK-31-6-353_F6.gif

    XRD patterns of samples.

    MRSK-31-6-353_F7.gif

    High-resolution XPS spectra for MoS2-fullerene.

    MRSK-31-6-353_F8.gif

    UV-vis absorption spectra of the samples.

    MRSK-31-6-353_F9.gif

    Photoluminescence spectra of MoS2 and the MoS2-fullerene composites with different fullerene contents.

    MRSK-31-6-353_F10.gif

    Degradation of MB under visible light for MoS2 (a), MoS2-C1 (b), MoS2-C2 (c) and MoS2-C3 (d).

    MRSK-31-6-353_F11.gif

    UV/Vis spectra of (a) TBS, (b) TMST and (c) DPCO concentration s after exposure to MoS2-fullerene composites under visbile light at various time intervals.

    MRSK-31-6-353_F12.gif

    Photo-Fenton degradated 50.0 mgL−1 NH3-N in 50.0 mL in solution. (1) 50.0 mgL−1 NH3-N + 0.50 g MoS2-C3 catalyst in the dark; (2) 50.0 mgL−1 NH3-N+0.50 g MoS2-C3 + 1.0 mmolL−1 H2O2 in the dark; (3) 50.0 mgL−1 NH3-N + visible light irradiation; (4) 50.0 mgL−1 NH3-N + 0.50 g MoS2-C3 + visible light irradiation; (5) 50.0 mgL−1 NH3-N + 1.0 mmolL−1 H2O2 + visible light irradiation; (6) 50.0 mgL−1 NH3-N + 0.50 g MoS2 + 1.0 mmolL−1 H2O2 catalyst + visible light irradiation; and (7) 50.0 mgL−1 NH3-N + 0.50 g MoS2-C3 + 1.0 mmolL−1 H2O2 catalyst + visible light irradiation.

    MRSK-31-6-353_F13.gif

    Effects of H2O2 concentration on the degradation of NH3.

    MRSK-31-6-353_F14.gif

    Schematic diagram of the separation of photogenerated electrons and holes on the photocatalyst interface.

    Table

    EDX elemental microanalysis and nomenclature of the samples

    Textural properties of original MoS2-fullerene composites.

    Reference

    1. Y. Ma, Z. H. Wang, Y. L. Jia, L. N. Wang, M. Yang, Y. X. Qi and Y. P. Bi, Carbon, 114, 591 (2017).
    2. D.E. Meeroff, F. Bloetscher, D.V. Reddy, F. Gasnier, S. Jain, A. McBarnette and H. Hamaguchi, J. Hazard. Mater., 209–210, 299 (2012).
    3. Y. C. Lin, W. J. Zhang, J. K. Huang, K. K. Liu, Y. H. Lee, C. T. Liang, C. W. Chu and L. J. Li, Nanoscale, 4, 6637 (2012).
    4. D. Q. Gao, M. S. Si, J. Y. Li, J. Zhang, Z. P. Zhang, Z. L. Yang and D. S. Xue, Nanoscale Res. Lett., 8, 129 (2013).
    5. H. Li, Z. Y. Yin, Q. Y. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang and H. Zhang, Small, 8, 63 (2012).
    6. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Chim, G. Galli and F. Wang, Nano Lett., 10, 1271 (2010).
    7. C. Lee, H. Yan, L. E. Brus, L. E. Heinz, T. F. Hone, J. Hone and S. Ryu, ACS Nano, 4, 2695 (2010).
    8. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nature Nanotechnol., 6, 147 (2011).
    9. G. L. Frey, S. Elani, M. Homyonfer, Y. Feldman and R. Tenne, Phys. Rev. B., 57, 6666 (1998).
    10. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett., 105, 136805 (2010).
    11. W. C. Oh, M. L. Chen, K. K. Y. Cho, C. L. Kim, Z. D. Meng and L. Zhu, Chinese J. Catal., 32, 1577 (2011).
    12. Z. D. Meng, L. Zhu, J. G. Choi, M. L. Chen and W. C. Oh, J. Mater. Chem., 21, 7596 (2011).
    13. D. Deutsch, J. Tarabek, M. Krause, P. Janda and L. Dunsch, Carbon, 42, 1137 (2004).
    14. J. J. Davis, H. A. O. Hill, A. Kurz, A. D. Leighton and A. Y. Safronov, J. Electroanal. Chem., 429, 7 (1997).
    15. J. Wang, Y. W. Guo, B. Liu, X. D. Jin, L. J. Liu, R. Xu, Y. M. Kong and B. X. Wang, Ultrason. Sonochem., 18, 177 (2011).
    16. A. A. Halim, H. A. Aziz, M. A. Megat, K. Shah and M. Nordin, J. Hazard. Mater., 175, 960 (2010).
    17. Q. Li, T. J. Newberg, E. C. Walter, J. C. Hemminger and R. M. Pender, Nano Lett., 4, 277 (2004).
    18. H. D. Wang, B. S. Xu, J. J. Liu, D. M. Zhuang, Mater. Chem. Phys., 91, 494 (2005).
    19. X. W. Zhang, M. H. Zhou and L. C. Lei, Carbon, 43, 1700 (2005).
    20. Q. Li, T. J. Newberg, E. C. Walter, J. C. Hemminger and R. M. Pender, Nano Lett., 4, 277 (2004).
    21. C. Ataca, H. Sahin, E. Akturk and S. Ciraci, J. Phys. Chem. C., 115, 3934 (2011).
    22. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli and F. Wang, Nano Lett., 10, 1271 (2010).
    23. A. Martinez-Alonso, J. M. D. Tascon and E. J. Bottani, J. Phys. Chem. B, 105, 135 (2001).
    24. R. Shidpour and M. Manteghian, Nanoscale., 2, 1429 (2010).
    25. H. Pan and Y. W. Zhang, J. Mater. Chem., 22, 7280 (2012).
    26. Y. F. Li, Z. Zhou, S. B. Zhang and Z. F. Chen, J. Am. Chem. Soc., 130, 16739 (2008).
    27. J. J. Chastain, R. C. King, Handbook of X-ray photoelectron spectroscopy, p. 157, USA: Physical Electronics, Inc., 1995.
    28. S. Suto, K. Sakamoto, D. Kondo, T. Wakita, A. Kimura and A. Kakizaki, Surf. Sci., 438, 242 (1999).
    29. R. S. Zhai, A. Das, C. K. Hsu, C. C. Han, T. Canteenvala, L. Y. Chiang and T. J. Chuang, Carbon, 42, 395 (2004).
    30. T. W. Odom, C. L. Stender, E. C. Greyson and Y. Babayan, Adv. Mater., 17, 2837 (2005).
    31. N. Kang, H. P. Paudel, M. N. Leuenberger, L. Tetard and S. I. Khondaker, J. Phys. Chem. C, 118, 21258 (2014).
    32. Y. J. Chen, G. H. Tian, Y. H. Shi, Y. T. Xiao and H. G. Fu, Appl. Catal., B, 164, 40 (2015).
    33. Z. D. Meng, T. Ghosh, L. Zhu, J. G. Choi, C. Y. Park and W. C. Oh, J. Mater. Chem., 22, 16127 (2012).
    34. D. L. Miles and C. Espejo, Analyst, 102, 104 (1977).
    35. X. Zhu, S. R. Castleberry, M. A. Nanny and E. C. Butler, Environ. Sci. Technol., 39, 3784 (2005).
    36. Y. Zhou, B. Xiao, S. Q. Liu, Z. D. Meng, Z. G. Chen, C. Y. Zou, C. B. Liu, F. Chen and X. Zhou, Chem. Eng. J., 283, 266 (2016).
    37. H. T. Lin, X. Y. Chen, H. L. Li, M. Yang and Y. X. Qi, Mater. Lett., 64, 1748 (2010).
    38. E. Goki, Y. Hisato, V. Damien, F. Takeshi, M. W. Chen and C. Manish, Nano Lett., 11, 5111 (2011).
    39. Z. D. Meng, M. M. Peng, L. Zhu and W. C. Oh, Appl. Catal. B, 113-114, 141 (2012).
    40. F. A. Frame and F. E. Osterloh, J. Phys. Chem. C, 114, 10628 (2010).