g-C 3 N 4 /MoS 2 Heterojunction for Photocatalytic Removal of Phenol and Cr(VI)

: In this study, molybdenum disulfide (MoS 2 ) decorated on graphitic carbon nitride (g-C 3 N 4 ) heterostructure catalysts at various weight ratios (0.5%, 1%, 3%, 10%, w / w ) were successfully prepared via a two-step hydrothermal synthesis preparation method. The properties of the synthesized materials were studied by X-ray diffraction (XRD), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FT-IR), UV–Vis diffuse reflection spectroscopy (DRS), scanning electron microscopy (SEM) and N 2 porosimetry. MoS 2 was successfully loaded on the g-C 3 N 4 form-ing heterojunction composite materials. N 2 porosimetry results showed mesoporous materials, with surface areas up to 93.7 m 2 g −1 , while determined band gaps ranging between 1.31 and 2.66 eV showed absorption over a wide band of solar light. The photocatalytic performance was evaluated towards phenol oxidation and of Cr (VI) reduction in single and binary systems under simulated sunlight irradiation. The optimum mass loading ratio of MoS 2 in g-C 3 N 4 was 1%, showing higher photocatalytic activity under simulated solar light in comparison with bare g-C 3 N 4 and MoS 2 for both oxidation and reduction processes. Based on scavenging experiments a type-II photocatalytic mechanism is proposed. Finally, the catalysts presented satisfactory stability (7.8% loss) within three catalytic cycles. Such composite materials can receive further applications as well as energy conversion.


Introduction
Energy crisis and environmental pollution are currently among the most serious problems for humans [1]. Emerging and priority environmental pollutants comprise several chemical categories such as pharmaceuticals, pesticides and plasticizers. Phenol and in general phenolics may be present in several wastewater streams from different production processes as well as degradation products from the previously mentioned pollutants. In addition, toxic metal ions such as Cr (VI) may be present in combined wastewater streams; thus, treatment methods should be capable of removing both organic and inorganic pollutants. However, conventional treatment methods present either low removal of certain organic pollutants or are incapable of simultaneous removal of organics and toxic metal ions [2]. Therefore, there has been an increased interest in the development of more efficient and environmentally friendly methods for the removal of pollutants from wastewater, and for this reason the advanced oxidation processes (AOPs) are widely used. Their effectiveness for organics removal is based on the production of hydroxyl radicals, which are powerful oxidizing agents. Heterogeneous photocatalysis, which presents advantages over other removal techniques as it can be applied for both oxidation and reduction of pollutants in ambient conditions in the presence of solar radiation and for organic pollutants, can lead to complete mineralization [2][3][4][5][6].
Graphitic carbon nitride (g-C3N4) has drawn widespread attention and become a hotspot in various scientific disciplines as a metal-free semiconductor due to its advantages. It has an appealing electronic structure with a 2.7 eV band gap, it can be fabricated with low-cost precursors (e.g., urea, thiourea, melamine) [3,7,8] and it is environmentally friendly. However, the rapidly photogenerated electron-hole recombination diminishes the photocatalytic efficiency. The coupling of g-C3N4 with other semiconductors with an appropriate band gap was considered to increase photocatalytic efficiency.
Molybdenum sulfide (MoS2) has attracted attention as an emerging photocatalytic cocatalyst material due to its distinctive features, such as high stability and hardness, reliability, non-toxicity and low cost. It possesses a narrow band gap (1.2-1.9 eV), which offers a wide spectral absorption range and provides high speed channels for the interfacial charge transfer in heterostructures [3]. Previous studies have explored the application of MoS2 in Cr(VI) reduction. Photoreduction and chemical reduction (redox between MoS2 and Cr(VI)) were determined through a collaborative contribution to reducing Cr(VI) by MoS2 under light conditions [9][10][11]. The coupling of two semiconductors (MoS2, g-C3N4) could significantly reduce the charge carrier recombination and highly improve photocatalytic activity compared to those bare materials. MoS2/g-C3N4 has been synthesized and tested for energy conversion, as a supercapacitor and for organic dye degradation [3,[12][13][14][15]; however, its application to the simultaneous oxidation/reduction of environmental pollutants has not been studied so far.
Based on the above, the present study deals with the preparation of a series of MoS2/g-C3N4 heterostructure composite catalysts and their application for the removal of phenol and Cr (VI) for aqueous solutions. There are few works exploring the synthesis of MoS2/g-C3N4 heterojunction structure via various methods such as hydrothermal treatment, impregnation-sulfidation and photodeposition [16][17][18][19]. Here, a two-step hydrothermal synthesis combined with a sonication-liquid exfoliation preparation method is proposed.

Preparation of g-C3N4, MoS2 and Composite MoS2/g-C3N4 Photocatalysts
2.2.1. Synthesis of Bare Carbon Nitride (g-C3N4) and Molybdenum Disulfide (MoS2) Graphitic carbon nitride (g-C3N4) was synthesized by thermal treatment of urea as a precursor compound. In a typical synthesis of g-C3N4, 30 g of urea was put in a crucible. After being dried at 90 °C it was heated to 500 °C at a heating rate of 10 °C/min and kept at this temperature for 4 h. The resulting yellow powder was collected and ground into a powder.
Molybdenum disulfide was prepared by hydrothermal synthesis. Hexaammonium heptamolybdate tetrahydrate and thiourea were dissolved in 10 mL of deionized water by stirring for 1 h at room temperature. The resulting solution was then placed in a Teflonlined autoclave reactor and heated in an oven at 200 °C for 12 h.

Synthesis of MoS2/g-C3N4 Heterostructure
MoS2/g-C3N4 heterostructures with 0.5%, 1%, 3%, 10% (wt%) ratios were synthesized. An appropriate amount of bulk g-C3N4 was added in isopropanol into a beaker, and the mixture was sonicated for 2 h at room temperature. In a separate beaker, MoS2 nanoparticles was added in a water/isopropanol (2:1) solution, and the mixture was sonicated for 2 h at room temperature. The solutions were mixed together and sonicated for another 2 h at room temperature. The product was cleaned with water and ethanol, collected by centrifuging and dried at 60 °C [18]. Then it was calcinated in air at 200 °C for 4h.

Material Characterization
The crystal forms of the photocatalysts were analyzed from their X-ray diffraction (XRD) patterns using a Bruker Advance D8 X-ray powder diffractometer (Billerica, MA, USA) working with Cu-Ka (λ = 1.5406 Å) radiation. Diffractograms were scanned from 2θ 10° to 90°. The patterns were assigned to crystal phases with the use of the International Center for Diffraction Data (ICCD).
Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FT-IR) was conducted to characterize the functional groups of different catalysts. A Shimadzu IR Spirit-T spectrophotometer (Kyoto, Japan) with a QATR-S single-reflection ATR measurement attachment equipped with diamond prism was used. The photocatalysts were scanned in the range 4000-400 cm −1 .
The UV-Vis diffuse reflectance spectra (DRS) of the catalyst powders were recorded using a Shimadzu 2600 spectrophotometer bearing an IRS-2600 integrating sphere (Kyoto, Japan) in the wavelength of 200-800 nm at room temperature using BaSO4 (Nacalai Tesque, extra pure reagent, Kyoto, Japan) as a reference sample.
The morphology of the catalysts was examined by scanning electron microscopy (SEM) using a JEOL JSM 5600 (Tokyo, Japan) working at 20 kV.
Nitrogen adsorption-desorption isotherms were collected by porosimetry using a Quantachrome Autosorb-1 instrument (Bounton Beach, FL, USA) at 77K. The samples (≈ 80 mg) were degassed at 150 °C for 3 h. The Brunauer-Emmet-Teller (BET) method was used to calculate the specific area (SSA) of each material at relative pressure between 0.05 and 0.3. The adsorbed amount of nitrogen at relative pressure P/P0 = 0.95 was used in order to calculate the total pore volume (VTOT). The Barrett, Joyner and Halenda (BJH) method was used to determine the pore size distribution (PSD) of the photocatalysts.

Photocatalytic Experiments
Photocatalytic experiments were carried out using Atlas Suntest XLS+ (Atlas, Germany) solar simulator apparatus equipped with a xenon lamp (2.2 kW) and special filters in place to prevent the transmission of wavelengths below 290 nm. During the experiments the irradiation intensity was maintained at 500 W m −2 .
Aqueous solutions (100 mL) and the catalyst (10 mg) were transferred into a doublewalled Pyrex glass reactor, thermostated by water flowing in the double-skin of the reactor. In binary systems the pH was adjusted to ca. 2 with sulfuric acid in order to achieve high adsorption efficiency of Cr(VI) onto the catalysts and avoid Cr(OH)3 deposition on the catalyst surface, based on previous studies [11,20]. Before exposure to light, the suspension was stirred in the dark for 30 min to ensure the establishment of adsorptiondesorption equilibrium onto the catalyst surface. The obtained aliquot (5 mL) was withdrawn at different time intervals and was filtered through 0.45 μm filters in order to remove the catalyst's particles before further analysis. Quantification of phenol for the kinetic studies in aquatic solutions was carried out by high performance liquid chromatography (HPLC) (Schimadzu, LC 10AD, Diode Array Detector SPD-M10A, Degasser DGU-14A). The column oven (Schimadzu, CTO-10A) was set at 40 °C. The mobile phase used was a mixture of water HPLC grade (50%) and methanol HPLC grade (50%). The concentration of Cr(VI) was determined by the diphenylcarbazide colorimetric method measuring the absorbance at the wavelength of 540 nm using a UV-Vis spectrophotometer (Jasco-V630, Tokyo, Japan).
For the prepared composite catalysts no diffraction peaks corresponding to MoS2 can be recognized, which may be because of the small quantity of MoS2 contents and high dispersion in g-C3N4 photocatalysts.

Morphology Surface Analysis
The morphological evaluation of all catalysts was carried out by scanning electron microscopy ( Figure 3). Figure 3a shows the morphology of g-C3N4 consisting of packed sheets. Figure 3b presents aggregation of flower-like spherical particles of MoS2 [26]. Figure 3c (1MoS2/g-C3N4) shows a morphology that seems to be close to that of g-C3N4; this may be because of the small quantity of MoS2 contents on the surface of g-C3N4. The 10MoS2/g-C3N4 (Figure 3d) reveals MoS2 spheres attached on the surface of the g-C3N4 sheets. The nitrogen adsorption-desorption isotherms for the prepared photocatalysts are presented in Figure 4.
All composites samples exhibited typical type IV(a) adsorption and desorption isotherms with H3 hysteresis loop according to the IUPAC classification, suggesting mesoporous materials [27]. BET specific surface area ranged between 2.0 m 2 g −1 for MoS2 and 93.7 m 2 g −1 for g-C3N4. MoS2 presented a non-porous structure with some porosity owed to the aggregation of spherical particles. The calculated specific surface area, average pore diameter and total pore volume of the catalysts are shown in Table 1.

UV-Vis Spectroscopy and Band Gap Determination
The diffuse reflectance spectroscopy (DRS) results and the energy band gap (Eg), calculated with the Kübelka-Munk function, of all photocatalysts are presented in Figure 5. The absorption edge of MoS2 was determined at 946 nm, while the absorption edge of g-C3N4 was at 480 nm indicating visible light response. The absorption edges of the prepared composite catalysts varied between 466-946 nm and the energy band gap 1.31-2.66 eV. The band gap energy and corresponding absorption edges are shown in Table 2.
MoS2/g-C3N4 heterojunction samples presented enhanced light absorption in the visible region as compared to g-C3N4. The absorption intensity of the prepared samples strengthened with increasing MoS2 contents, which agrees with the color of the prepared samples that vary from light yellow to grey.
The band edge position of conduction band (CB) and valence band (VB) of prepared catalysts was determined applying Equations (1) and (2).
where EVB is the VB edge potential, ECB is the CB edge potential, Eg is the band gap energy of the catalysts, X is the electronegativity of g-C3N4 (4.67 eV) and MoS2 (5.33 eV) [28] and E0 is the energy of free electrons on the hydrogen scale (4.5 eV). The EVB and ECB of g-C3N4 was calculated at 1.46 and −1.12 eV, respectively, while for MoS2 the corresponding values were 1.48 and 0.17 eV.

Photocatalytic Study
The photocatalytic performance of composite catalysts was studied against the oxidation of phenol (single system: Cph = 10 mg L −1 , Ccat = 100 mg L −1 ) as well as the simultaneous photocatalytic reduction of Cr(VI) and oxidation of phenol (binary system: phenol/Cr(VI) = 1/10, Ccat = 100 mg L −1 , pH = 2 [29,30]) under simulated sunlight irradiation. The results of all photocatalytic experiments are shown in Figure 6 and Tables 3 and 4.
In comparison, the prepared catalysts exhibit faster kinetics of phenol and Cr(VI) removal in binary than in single systems.   Table 4. Kinetic parameters (first-order kinetic constants (k), half-life (t1/2) and correlation coefficients (R 2 )) of phenol and Cr(VI) in binary system.

Photocatalytic Mechanism for the Composite Photocatalysts
Different photoinduced reactive species, such as hydroxyl radicals (OH . ), superoxide radicals (O2 − ) and holes (h + ), may be involved in the photocatalytic process after electronhole pairs are generated. Thus, photocatalytic experiments in the presence of isopropanol (IPA), superoxide dismutase (SOD) and triethanolamine (TEOA) were used to scavenge • OH, O2 − and h + , respectively, during the photodegradation of phenol in order to investigate the photocatalytic mechanism [3]. As shown in Table 5, the degradation rate constant (k) for phenol was 0.009 min −1 after exposure for 120 min under simulated sunlight without scavenger addition, and this changed after adding, IPA, SOD and TEOA, to 0.003, 0.004 and 0.002 min −1 , respectively. It is clearly observed that the degradation of phenol was inhibited with the addition of scavengers, though TEOA was the most obvious. The degradation kinetics of phenol in the presence of scavengers are shown in Figure 7 and Table 5.  According to the above analysis, the proposed photocatalytic mechanism (Type II), involving the carrier transfer and photodegradation of phenol was illustrated as shown in Figure 8. The photogenerated electron-hole pairs were induced in the conduction band (CB) and valence band (VB) of g-C3N4 and MoS2 under irradiation. The photogenerated electrons transferred from the CB of g-C3N4 to that of MoS2, while holes moved from the VB of MoS2 to that of g-C3N4, leading the photogenerated electron-hole pair to be separated and transferred effectively. The holes (h + ) in the VB of g-C3N4 participate in the degradation process. Simultaneously, electrons (e − ) in the CB together with H + can reduce O2 to H2O2, which forms hydroxyl radicals. The eon the CB of g-C3N4 and MoS2 participates in the reduction of Cr (VI) to Cr(III). The proposed mechanism provides additional explanation for the better removal performance in the binary system. Regarding the reduction process, the electrons on MoS2 have weaker reduction ability; consequently they can reduce strong oxidation agents such as Cr(VI), enhancing the target reduction reaction without other reactions, such as O2 reduction. As far as the oxidation pathway is concerned, the enhancement is due to the better charge separation. The results of the present study indicate that the MoS2 could be a promising candidate as a non-noble metal co-catalyst for g-C3N4 photocatalysts. The composite catalysts can receive further applications either for degradation of pollutants or in energy conversion.

Recyclability of the Composite Photocatalysts
The stability of the prepared photocatalysts was investigated by examining the photocatalytic activity of the most efficient photocatalyst (1MoS2/g-C3N4) against oxidation of phenol in binary system for three continuous cycles (Figure 9). Reaction constants of phenol oxidation were 0.090, 0.089 and 0.083 min −1 for the first, second and third cycle, respectively (Table 6). A total loss of photocatalytic activity about 7.8% after the third photocatalytic cycle showed the sufficient stability of the catalyst considering also the possible losses of catalyst particles during the recovery process after each cycle. In addition, the catalyst separated at the end of the third catalytic cycle was characterized by XRD, SEM and ATR-FT-IR (Figure 10a-c), revealing a nearly identical pattern to the starting material. Figure 9. Reusability performance of 1MoS2/g-C3N4 photocatalyst for three continuous cycles. Table 6. Kinetic parameters (first-order kinetic constants (k), half-life (t1/2) and correlation coefficients (R 2 )) of phenol degradation by 1MoS2/g-C3N4 catalyst after three consecutive cycles.

Conclusions
In this work, mesoporous MoS2/g-C3N4 heterostructures were successfully synthesized via a two-step hydrothermal synthesis at various weight ratios (up to 10% w/w of MoS2) and assayed for the simultaneous oxidation of phenol and Cr(VI) reduction. Composite materials show higher photocatalytic activity under solar light in comparison with bare g-C3N4 and MoS2 for both oxidation and reduction processes, while the material with 1% MoS2 loading was found as the catalyst with the best efficiency. A considerable enhancement of the photocatalytic efficiency was observed for the binary system compared to the single component systems, indicating a promoting synergistic effect. The three cycling experiments indicated that the catalysts presented a good stability. Finally, type-II band alignment was proposed as the photocatalytic mechanism for the composed catalysts.