MoS2 and Fe2O3 co-modify g-C3N4 to improve the performance of photocatalytic hydrogen production

Photocatalytic hydrogen production as a technology to solve energy and environmental problems exhibits great prospect and the exploration of new photocatalytic materials is crucial. In this research, the ternary composite catalyst of MoS2/Fe2O3/g-C3N4 was successfully prepared by a hydrothermal method, and then a series of characterizations were conducted. The characterization results demonstrated that the composite catalyst had better photocatalytic performance and experiment results had confirmed that the MoS2/Fe2O3/g-C3N4 composite catalyst had a higher hydrogen production rate than the single-component catalyst g-C3N4, which was 7.82 mmol g−1 h−1, about 5 times higher than the catalyst g-C3N4 (1.56 mmol g−1 h−1). The improvement of its photocatalytic activity can be mainly attributed to its enhanced absorption of visible light and the increase of the specific surface area, which provided more reactive sites for the composite catalyst. The successful preparation of composite catalyst provided more channels for carrier migration and reduced the recombination of photogenerated electrons and holes. Meanwhile, the composite catalyst also showed higher stability and repeatability.

With the rapid development of the social economy, human demand for energy is increasing. Traditional fossil energy is non-renewable, and the extensive use of fossil resources brings a serious crisis to the environment 1-3 . Therefore, it is urgent to find a kind of alternative energy to replace fossil energy. Hydrogen as a clean and renewable energy has received widespread attention. Photocatalytic hydrogen production is a technology that uses solar energy to produce hydrogen, exhibits broad prospect in solving resource and environmental pollution problems 4 . However, in the current photocatalysis research, the semiconductor catalysts show a low rate of hydrogen generation due to their various defects and cannot be widely used. Therefore, the development of efficient and stable photocatalysts has become a hot spot in current research.
In the past few decades, scholars have devoted themselves to finding semiconductor catalytic materials with high catalytic performance, semiconductor materials have been extensively explored, such as TiO 2 5-7 , CdS 8-10 , ZnO [11][12][13] and g-C 3 N 4 [14][15][16] . Among the various semiconductor catalysts that have been currently developed and researched, g-C 3 N 4 , as a non-metal semiconductor material, has attracted wide attention because of its suitable band gap, good stability, and visible light response. However, its small specific surface area and strong photogenerated electron-hole recombination ability hinder its hydrogen production performance [17][18][19] . Many methods have been adopted in the current research to improve the performance of the semiconductor catalyst g-C 3 N 4 and increase its hydrogen production rate. For example, using doped metal and non-metal elements to accelerate the separation of charges [20][21][22][23] , changing the structure of g-C 3 N 4 to increase its specific surface area 24 , forming a heterojunction with other materials [25][26][27] , or supporting cocatalysts to accelerate electron transfer 28 .
Hematite (α-Fe 2 O 3 ), as a transition metal oxide with a band gap of 2.0-2.2 eV, has been explored extensively. Because of its various forms, high photocurrent density and good thermodynamic stability, it was used widely in photocatalysis research 29 . A lot of studies have been conducted to form a heterojunction between Fe 2 O 3 and g-C 3 N 4 30-32 , and the results confirmed that when the heterojunction was formed, the electrons of Fe 2 O 3 can be transferred to the conduction band of g-C 3 N 4 to consume holes under light conditions, thus reducing the electron-holes recombination in g-C 3 N 4 to improve the catalytic activity of composite catalysts. MoS 2 is a transition metal sulfide with a band gap of about 1.9 eV, has a wide spectral absorption range and can provide migration channels for carriers 33  Preparation of g-C 3 N 4 : In general, melamine was used as a precursor in a muffle furnace to synthesize g-C 3 N 4 in an air atmosphere. 5 g melamine was put in a crucible with a lid and calcined at 550 °C for 4 h, with a heating rate of 2.5 °C/min. After the calcination was completed, it was cooled and ground the obtained lump g-C 3 N 4 to obtain light yellow powder, denoted as CN.
Preparation of MoS 2 : Typically, 1.21 g Na 2 MoO 4 ·2H 2 O and 1.56 g L-cysteine were dissolved in 50 mL deionized water, stirred for 30 min to fully dissolve the solid. Then the solution was diverted to an autoclave and reacted at 220 °C for 24 h. The sample was separated by centrifugation and then washed three times with deionized water. The obtained black solid was dried in an oven at 65 °C and named MoS 2 .
Preparation of MoS 2 /Fe 2 O 3 /g-C 3 N 4 : MoS 2 /Fe 2 O 3 /g-C 3 N 4 composite catalyst was prepared by hydrothermal method. Typically, 500 mg CN and 5 mg MoS 2 were dissolved in 50 mL deionized water, 25 mg Fe(NO 3 ) 3 ·9H 2 O solution and 10 mL (NH 4 ) 2 CO 3 (0.5 M) solution were added in it, then the above suspension was sonicated for 30 min and transferred to an autoclave before it was hydrothermally heated at 180 °C for 12 h. When the autoclave cooled down to room temperature, the material thus obtained was separated by centrifugation and then washed three times with deionized water and dried in an oven at 65℃. Then the obtained material was ground, and the obtained light red powder was named 1%MoS 2  Characterization. The structures of samples were determined on an X-ray diffractometer (XRD, 18KW/D/ max2550VB/PC), the scanning range from 10°-80°. The FT-IR spectra were obtained in the range of 7800-350 cm −1 . A field electron emission microscope (SEM, Gemini SEM 500) and transmission electron microscope (TEM, JEM-1400) were applied to characterize the structures of the catalysts. The dates of X-ray photoelectron spectroscopy (XPS) were obtained on ESCALAB 250 (Thermo, USA) equipped with an Al Kα radiation source. The specific surface area and pore size distribution were measured through nitrogen adsorption-desorption (Angstroms/3Flex, USA) at 77 K. The UV-Vis absorption spectra ranging from 200 to 800 nm were determined by a Scan UV-Vis spectrophotometer (America, Lambda 950). The photoluminescence (PL) spectra were acquired by Fluorescence Spectrofluorometer. An electrochemical station was used to measure photocurrent responses and electrochemical impedance spectra.
Photocatalytic hydrogen production. The visible-light-induced H 2 release was measured in a closed photocatalytic online detection cycle system (CEL-SPH2N, Au Light, Beijing). A 300 W (λ > 400 nm) xenon lamp was selected as the visible light source. 0.05 g catalyst was stirred and dispersed in a glass container containing 20 mL sacrificial agent-Lactic acid and 80 mL distilled water. 2wt% Pt co-catalyst was photodeposited on the as-synthesized catalyst by using H 2 PtCl 6 ·6H 2 O as a precursor. The photocatalytic reaction system was evacuated before starting the reaction and the temperature of the reaction solution was maintained at 6 °C by the reaction of cooling water during the photocatalytic reaction. Using nitrogen as the carrier gas, the produced gas was analyzed by a gas chromatograph (GC7900) equipped with a thermal conductivity detector (TCD).  The specific morphology of the catalyst was of great significance to the study of its properties. The microscopic morphology of CN and MoS 2 /Fe 2 O 3 /CN was acquired by SEM and the results were illustrated in Fig. 3. Figure 3a-c displayed the morphology and structure of CN at different multiples. CN was an irregular block structure, which was consistent with the literature report 39,40 . And the block structure of CN led to its small specific surface area, which was also a significant reason for its low catalytic efficiency. Figure 3d-f showed the morphology and structure of MoS 2 /Fe 2 O 3 /CN at different multiples. The structure of the composite catalyst changed during the formation process, from the block structure to the rod structure, which was mainly attributed to the hydrothermal reaction. There were some small particles on the rod-shaped structure, but it was hard to judge whether there were traces of    The XPS spectrum of S 2p was displayed in Fig. 5d, the characteristic peaks at 161.3 eV and 162.6 eV have corresponded to S 2p 3/2 and S 2p 1/2 of S 2− . The peak at 168.31 eV belonged to the edge unsaturated S atom of the ultra-thin layer, which was considered to be the active site for hydrogen production and was the unique flake modification advantage of MoS 2 47 . The unique advantage of MoS 2 lied in the presence of edge unsaturated S atom, which provided more reaction site to improve the hydrogen production. The binding energy corresponding to the unsaturated S atom was 168.31 eV. where α, hν, and E g refers to absorption coefficient, photon energy and band gap, respectively. A is a constant, n = 1 means a direct band gap while n = 4 refers to an indirect band gap 48 . According to the equation, the band gaps of CN, Fe 2 O 3 /CN and MoS 2 /Fe 2 O 3 /CN can be roughly estimated, the results were shown in Fig. 6b. According to the formula, the band gap of CN was predicted to be 2.84 eV. Besides, the band gap values of the composite catalysts 5%Fe 2 O 3 /CN and 1%MoS 2 /Fe 2 O 3 /CN were 2.76 eV and 2.79 eV according to the prediction. The band gap of the synthesized composite catalysts were reduced, which were beneficial to the catalyst to further absorb sunlight and improve the photocatalytic performance. Table 1 described the specific surface area of CN, 5%Fe 2 O 3 /CN and 1%MoS 2 /Fe 2 O 3 /CN. As illustrated, the specific surface area of 1%MoS 2 /Fe 2 O 3 /CN was 61.76 m 2 /g, which was the highest among all catalysts, consistent with the results of the highest H 2 production rate discussed below. The specific surface area of the composite catalyst was increased mainly due to the change of the composite catalyst in structure, from the original block structure to a rod-like structure during the hydrothermal reaction process. This structure can provide more  www.nature.com/scientificreports/ 5%Fe 2 O 3 /CN and 1%MoS 2 /Fe 2 O 3 /CN was type IV, and the types of hysteresis loop were H3. As depicted, when the relative pressure was in the range of 0.8-1.0, the adsorption-desorption isotherm appears higher absorption, which meant that the catalyst had a porous structure.

Results and discussion
To compare the carrier separation efficiency of different photocatalysts, CN, 5%Fe 2 O 3 /CN and 1%MoS 2 / Fe 2 O 3 /CN were characterized by photoluminescence spectroscopy and time-resolved PL spectra, the results were described in Fig. 8a, b. The PL spectrum can display the photogenerated electron-hole separation ability of the prepared composite catalyst. Generally, the stronger the spectrum intensity of the PL, the lower the separation efficiency of the photocatalyst carriers. The PL spectrum of pure CN had very high fluorescence intensity in the range of 420-550 nm, which was about 4 times higher than the composite catalyst MoS 2 /Fe 2 O 3 /CN. It meant that the pure CN had weaker carrier mobility than the composite catalyst, and photo-generated electron-hole was more likely to occur complex. The fluorescence intensity of the MoS 2 /Fe 2 O 3 /CN composite catalyst was the weakest, indicating it had the highest carrier separation efficiency and lowest probability of photo-generated electron-hole recombination among all photocatalysts. Time-resolved PL spectroscopy further demonstrates the characteristics of charge carrier lifetime. The average lifetime of MoS 2 /Fe 2 O 3 /CN was 4.954 ns, which shorter than CN (5.717 ns) and Fe 2 O 3 /CN (5.538 ns), it meant MoS 2 /Fe 2 O 3 /CN can effectively inhibit the recombination    www.nature.com/scientificreports/ of electron and hole. The PL and time-resolved PL spectra results were consistent with the conclusion of the hydrogen production test. Under visible light condition, the hydrogen production rate of MoS 2 /Fe 2 O 3 /CN composite catalyst was shown in Fig. 9, including the control groups. The hydrogen production rate of pure CN was 1.56 mmol g −1 h −1 , while the rates of Fe 2 O 3 and MoS 2 were relatively low, which can be almost ignored. Fe 2 O 3 /CN composite catalyst demonstrated enhanced hydrogen release performance and the 5%Fe 2 O 3 /CN photocatalyst showed the best hydrogen production rate, which was 5.65 mmol g −1 h −1 . After loading MoS 2 on the Fe 2 O 3 /CN composite catalyst, the photocatalytic hydrogen production rate of the composite catalyst was further improved. The highest hydrogen production rate of 1%MoS 2 /Fe 2 O 3 /CN composite catalyst was 7.82 mmol g −1 h −1 , which was about 5 times higher than that of pure CN (1.56 mmol g −1 h −1 ) and 1.38 times higher than 5%Fe 2 O 3 /CN. The reusability and stability of the catalyst were considered to be important practical evaluation and application parameters. In this paper, using MoS 2 /Fe 2 O 3 /CN as the photocatalyst, five consecutive cycles of experiments were conducted to explore the stability of hydrogen evolution. As displayed in Fig. 9c, the hydrogen generation rate was still stable after five cycles, indicating the good stability and sustainable utilization of the photocatalyst.
Electrochemical characterization. An electrochemical impedance spectroscopy test was carried out to evaluate the charge transfer capability of the photocatalyst, and the results were demonstrated in Fig. 10. As described in Fig. 10, the resistance of pure CN was relatively large, which hindered its charge transfer. After being combined with 5% Fe 2 O 3 , the resistance of the composite catalyst was greatly decreased. When the catalyst was further combined with MoS 2 , the charge transfer resistance of the composite catalyst was further decreased. It was known that when the resistance of electrochemical impedance was smaller, the resistance of charge transfer was lower, the photocatalytic activity was better. The results of PL demonstrated that the charge transferability of the MoS 2 /Fe 2 O 3 /CN composite catalyst was improved, thus the photocatalytic performance was improved.
The test of transient photocurrent response was an effective way to evaluate the carrier mobility of photocatalysts. And the photocurrent responses diagrams of CN, 5%Fe 2 O 3 /CN and 1%MoS 2 /Fe 2 O 3 /CN photocatalysts were shown in Fig. 11. The photocurrent responses current of the composite catalyst 1%MoS 2 /Fe 2 O 3 /CN was significantly higher than the catalysts of 5%Fe 2 O 3 /CN and CN, which implied that its electron-hole separation efficiency was the highest among the samples. And the results also illustrated that the photocatalytic activity of the composite catalyst was the best, which was consistent with the results of PL and EIS.
Photocatalytic mechanism. In order to explore the process mechanism of photocatalytic hydrogen production, the EPR characterization technology was used to detect the free radicals in the photocatalytic process. The test results were shown in Fig. 12 and the EPR signals of ·OH and ·O 2 − were shown in Fig. 12a, b, respectively. The results demonstrated that signals of ·OH and ·O 2 − were existed in the system of MoS 2 /Fe 2 O 3 /CN and the peak intensity of MoS 2 /Fe 2 O 3 /CN was the highest, which indicated that the catalytic mechanism of the ternary composite catalyst can be represented by Z-scheme. Based on the EPR and HRTEM results, the possible photocatalytic mechanism of the MoS 2 /Fe 2 O 3 /CN composite catalyst was illustrated in Fig. 13. The band gap structures of g-C 3 N 4 , Fe 2 O 3 and MoS 2 were determined by previous studies 49,50 . Under the condition of light, electrons were generated in the valence bands of CN、Fe 2 O 3 and MoS 2 , the electrons transferred from the valence band to the conduction band, and holes were generated in the valence band. It was known from the literature that if the reaction was based on the traditional heterojunction electron transfer mechanism, the active materials ·OH and ·O 2 − cannot be produced due to the potential of CN、Fe 2 O 3 and MoS 2 46 , which was inconsistent with the EPR test results in Fig. 12. Therefore, based on the previously reported literature, in this paper, we explained the photocatalytic performance of the composite catalyst through the Z-type heterojunction mechanism. In this system, the electrons in the Fe 2 O 3 conduction band were transferred to the valence band of CN under the action of the intermediary MoS 2 , which consumed the holes and reduced the photo-generated electron-hole recombination in CN. The sacrificial agent lactic acid was oxidized on the valence band of Fe 2 O 3 , consuming holes and   www.nature.com/scientificreports/ accelerating the migration of carriers. Under the action of the co-catalyst MoS 2 and Pt, the active sites of CN can effectively produce hydrogen.

Conclusions
In conclusion, the ternary composite catalyst MoS 2 /Fe 2 O 3 /CN was successfully prepared through the hydrothermal method, and a series of characterizations confirmed that the composite catalyst has the superior photocatalytic performance among the samples. The various characterization results demonstrated that with the addition of Fe 2 O 3 and MoS 2 , the ternary composite catalyst MoS 2 /Fe 2 O 3 /CN had a larger specific surface area, stronger visible light absorption capacity, higher carrier migration efficiency, and lower photogenerated electron-hole recombination rate. And MoS 2 /Fe 2 O 3 /CN showed higher hydrogen production activity, reaching 7.82 mmol g −1 h −1 , which was about 5 times higher than the basic catalyst CN. The Z-scheme photocatalytic process was testified by EPR analysis and the research on the composite catalyst MoS 2 /Fe 2 O 3 /CN provided some useful reference information for the development of other ternary composite catalysts.