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Promoting Charge Separation in g-C3N4/Graphene/MoS2 Photocatalysts by Two-Dimensional Nanojunction for Enhanced Photocatalytic H2 Production Yong-Jun Yuan, Yan Yang, Zijian Li, Daqin Chen, Shiting Wu, Gaoliang Fang, Wangfeng Bai, Mingye Ding, Ling-Xia Yang, Dapeng Cao, Zhentao Yu, and Zhigang Zou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00030 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Energy Materials

Promoting Charge Separation in g-C3N4/Graphene/ MoS2 Photocatalysts by Two-Dimensional Nanojunction for Enhanced Photocatalytic H2 Production Yong-Jun Yuan,*,† Yan Yang,† Zijian Li,† Daqin Chen,* ,† Shiting Wu,† Gaoliang Fang,† Wangfeng Bai,† Mingye Ding,† Ling-Xia Yang,‡ Da-Peng Cao,§ Zhen-Tao Yu,*,‡, Zhi-Gang Zou‡,∥ †

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou

310018, People’s Republic of China. ‡

National Laboratory of Solid State Microstructures and Collaborative Innovation Center of

Advanced Microstructures, College of Engineering and Applied Science, Nanjing University, Nanjing 210093, People’s Republic of China. §

College of Materials Science and Engineering, Nanjing University of Posts and

Telecommunications, 210023, People’s Republic of China. Macau Institute of Systems Engineering, Macau University of Science and Technology, Macau 9 99078, People’s Republic of China. *Corresponding authors: E-mail addresses: [email protected]; [email protected] or [email protected] Fax: (+ 86)-0571-87713538

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ABSTRACT. Graphitic carbon nitride (g-C3N4) is a promising photocatalyst for solar H2 generation, but the practical application of g-C3N4 is still limited by the low separation efficiency of photogenerated charge carrier. Herein, we report the construction of ternary gC3N4/graphene/MoS2 two-dimensional nanojunction photocatalysts for enhanced visible light photocatalytic H2 production from water. As demonstrated by photoluminescence and transient photocurrent studies, the intimate two-dimensional nanojuction can efficiently accelerate the charge transfer, resulting in the high photocatalytic activity. The g-C3N4/graphene/MoS2 composite with 0.5% graphene and 1.2% MoS2 achieves a high H2 evolution rate of 317 µmol h-1 g-1, and the apparent quantum yield reaches 3.4% at 420 nm. More importantly, the ternary gC3N4/graphene/MoS2 two-dimensional nanojunction photocatalysts exhibits much higher photocatalytic activity than the optimized Pt-loaded g-C3N4 photocatalyst.

Keywords: interface engineering, artificial photosynthesis, two-dimensional nanojunction, hydrogen production, carrier separation.


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The sunlight driven generation of the carbon-free H2 from water splitting using semiconductor photocatalysts represents a promising strategy to convert solar energy to chemical energy for sustainable development.1-3 Since the pioneering work of photolysis of water on TiO2 electrodes by Fujishima and Honda,4 numerous semiconductors have designed and developed as photocatalysts for photocatalytic H2 production from water during the past 40 years, such as TiO2,5 Cu2O,6 SrTiO3,7 CdS,8,9 ZnIn2S4,10 CuInS2,11 CdSe,12 ZnO:GaN,13 and g-C3N etc.14 Among these different semiconductors, graphitic carbon nitride (g-C3N4) is one of the most promising photocatalyst for visible light photocatalytic H2 generation owing to its relatively narrow band gap of 2.8 eV, suitable band edge positions, cost effectiveness, low toxicity as well as excellent durability.15-18 However, the intrinsic property of rapid recombination of photogenerated electron-hole pairs limits the photocatalytic activity of g-C3N4 for H2 generation.15 An efficient strategy to overcome the above drawback is that loading a suitable cocatalyst on g-C3N4 to provide abundant active sites, which could not only accelerate the transfer and migration of photgenerated charge carriers, but also lower the activation potential for H2 evolution reaction. Especially, noble metals such as Pt,19 Pd,20 and Au etc.,21 can act as efficient cocatalysts to enhance the photocatalytic performance of g-C3N4. However, noble metals are rare and expensive, thus it is of great significance to develop of noble-metal-free cocatalysts. Recently, numerous low-cost transition-metal based cocatalysts including molybdenum, nickel and cobalt compounds have been developed for g-C3N4-based photocatalytic hydrogen production systems.22-29 Among these transition-metal compounds, MoS2 has been proven to be an efficient cocatalyst owing to abundant exposed edges and low overpotential for H2 evolution reaction.25 Unfortunately, the poor electrical conductivity of MoS2 restricts it catalytic activity.


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Recent studies showed that the electrical conductivity and catalytic activity of MoS2 can be improved by coupling MoS2 with graphene due to the superior charge transfer ability of graphene.30-33 More importantly, both MoS2 and graphene have the same layered structure as gC3N4, which achieves a well-defined two-dimensional (2D) junction. The construction of ternary g-C3N4/RGO/MoS2

photocatalysts

can

provide

large

and

intimate

two-dimensional

nanojunctions, which would play a key factor in determining the charge separation efficiency and photocatalytic performance. Accordingly, it is expected that the combination of layered gC3N4 and 2D graphene/MoS2 composite cocatalyst is promising to develop a highly-efficient, earth-abundant and durable photocatalyst for visible-light-driven H2 production from water. Herein, we present the synthesis and characterization of ternary g-C3N4/reduced graphene oxidate/MoS2 (g-C3N4/RGO/MoS2) 2D nanojunction photocatalysts for enhanced photocatalytic H2 evolution. Such a smart architecture of g-C3N4/RGO/MoS2 photocatalysts provides a relatively short diffusion distance for efficient charge transfer, a large contact interface for rapid photogenerated charge separation, as well as abundant active sites for photocatalytic H2 evolution reactions. Therefore, the 2D g-C3N4/RGO/MoS2 nanojunction photocatalysts are anticipated to possess excellent photocatalytic performance for H2 production under visible light. The illustration of the preparation of g-C3N4/RGO/MoS2 photocatalysts is shown in Figure 1a. In the first step, the RGO incorporated g-C3N4 composite was first prepared through pyrolysis of urea on the surface of graphene oxide (GO) at 550 °C under an argon atmosphere. During this process, the GO was reduced to RGO and then attached on the surface of g-C3N4. Subsequent hydrothermal treatment of g-C3N4/RGO hybrid and (NH4)2MoS4 in a dimethyl formamide solution led to the crystallization of MoS2 and formation of g-C3N4/RGO/MoS2 samples. In these as-prepared samples, the mass fraction of RGO was 0.5%, and the mass fraction of MoS2 has a


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range from 0.4% to 2.0% (denoted as CGM(x), which contains 0.5% RGO and x% MoS2). Inductively coupled plasma-atomic emission spectroscopy (ICP-OES) was used to measure the amount of MoS2 exhibited in the CGM samples. The mass fraction of MoS2 in the as-synthesized CGM(0.4), CGM(1.2) and CGM(2.0) samples was determined to be about 0.35%, 1.09% and 1.87% (Table S1), respectively. The measured amounts of MoS2 present in the composites are close to the theoretical values. The as-prepared composites as well as bare g-C3N4, RGO and MoS2 were then characterized by X-Ray diffraction (XRD). As shown in Figure 1 b, the bare MoS2 exhibits four main diffraction peaks at 2θ = 14.2o, 33.42o, 39.84o and 59.32o, which can be ascribed to the (002), (100), (103) and (110) planes of hexagonal MoS2, respectively.34 As for the bare RGO sample, a broad diffraction peak centered at 2θ = 22.9o was observed, suggesting a

Figure 1. (a) Synthetic route for 2D g-C3N4/RGO/MoS2 nanojunction photocatalysts. (b) XRD patterns of g-C3N4, RGO, g-C3N4/RGO (0.5% RGO) and g-C3N4/RGO/MoS2 composites. (c) Raman spectra of g-C3N4, g-C3N4/RGO (0.5% RGO) and g-C3N4/RGO/MoS2 [CGM(1.2)] composite. (d) UV-Vis diffuse reflectance spectra of g-C3N4, g-C3N4/RGO (0.5% RGO) and gC3N4/RGO/MoS2 composites.


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random packing of graphene sheets in the bare RGO.31 The bare g-C3N4 shows two distinct diffraction peaks at 13.5o and 27.8 o, which can be attributed to the inter-planar stacking (002) peak of conjugated aromatic systems and in-plane structural packing motif (100) peak, respectively.31 It is noted that no characteristic diffraction peaks of both RGO and MoS2 were observed in all g-C3N4/RGO/MoS2 composites, which is related to the relatively low diffraction intensity and low contents of both MoS2 and RGO. To confirm the presence of both RGO and MoS2 in these as-prepared g-C3N4/RGO/MoS2 composites, Raman analysis was performed. As shown in Figure 1c, the characteristic Raman peaks of both MoS2 (E2g and A1g modes) and RGO (D and G bands) were observed in the Raman spectra of g-C3N4/RGO/MoS2 composite, indicating that both RGO and MoS2 were grown on the surface of g-C3N4 successfully.35 Figure 1d shows the UV-Vis diffuse reflectance spectra of bare g-C3N4, g-C3N4/RGO and gC3N4/RGO/MoS2 composites with different contents of MoS2. The bare g-C3N4 sample shows a strong absorption in UV region with an absorption edge at around 438 nm, corresponding to a band gap of 2.8 eV. After loading RGO/MoS2 cocatalyst on g-C3N4, no absorption edge shift was observed as compared to that of bare g-C3N4, suggesting the band structure of g-C3N4 was not affected after the introduction of RGO/MoS2 cocatalyst. However, the absorption in visible light region (440-800 nm) increases with increasing amounts of RGO/MoS2 cocatalyst, suggesting an extended visible light response resulting from the loading of RGO/MoS2 cocatalyst. Nitrogen adsorption-desorption isotherms (Figure S1) show that the bare g-C3N4 and CGM(1.2) sample has a Brunauer-Emmett-Teller (BET) surface area of 12.5 and 13.1 m2/g, respectively. The pore size of both bare g-C3N4 and CGM(1.2) sample is mainly less than 25 nm (inset in Figure S1). The BET surface area and average pore size of CGM(1.2) sample are close to that of bare g-C3N4, which could be attributed to the small amount of RGO/MoS2 cocatalyst.


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To further obtain the structural information of g-C3N4/RGO/MoS2 samples, TEM analysis was carried out. As shown in Figure 2a and Figure S2, the TEM images of g-C3N4/RGO/MoS2 sample give a general view of the morphology of the product over a large area. The composite has a typical layer structure with a clear surface. Notably, the HRTEM images shown in Figure 2b and c display clear lattice fringes of ca. 0.34 nm and 0.62 nm, which can be ascribed to the (001) plane of graphene and the (002) plane of hexagonal MoS2, respectively.32 In the HRTEM images, the 2D MoS2 and graphene were grown on the surface of g-C3N4, forming intimate 2D nanojunction. The unique 2D nanojunction could play a crucial factor in determining the photocatalytic H2 evolution performance of g-C3N4/RGO/MoS2 samples. In addition, it is also possible that small amount of MoS2 could also grow on the surface of g-C3N4.

Figure 2. (a,b) TEM images of g-C3N4/RGO/MoS2 [CMG(1.2)] sample; (c) HRTEM image of gC3N4/RGO/MoS2 [CMG(1.2)] sample. (d) Survey XPS spectrum of g-C3N4/RGO/MoS2 [CMG(1.2)] sample. (e) High-resolution XPS spectrum of Mo 3d. (f) High-resolution XPS spectrum of S 2p.


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X-Ray photoelectron spectroscopy was explored to determine the composition of these as-prepared g-C3N4/RGO/MoS2 composites. As illustrated in Figure 2d, the survey spectrum of the g-C3N4/RGO/MoS2 composite confirms the coexistence of C (284.4 eV), N (402.3 eV), Mo (230.2 eV), and S (162.3 eV) elements as excepted. The evolution of Mo 3d with two weak peaks was observed at 228.4 and 232.1 eV (Figure 2e), which can be attributed to Mo 3d5/2 and Mo 3d3/2, respectively.36 Meanwhile, a weak peak for Mo 3d at 235.8 eV was also observed, suggesting the presence of Mo6+, presumably due to the formation of small content of surface oxide species. The high-resolution S 2p at binding energies of 161.7 and 162.8 eV correspond to S 2p3/2 and S 2p1/2, respectively (Figure 2f).34 The Mo/S atomic ratio was observed to be 1:1.91, which is very closed the nominal composition of MoS2. The XPS analysis of g-C3N4/RGO/MoS2 composite reveals the presence of C, N, Mo, and S elements. The C 1s binding energy of gC3N4/RGO/MoS2 composite shows two mainly peaks with binding energies of 288.4 and 284.5 eV (Figure S3), which can be assigned to C-N-C groups of g-C3N4 and C-C bond of graphene, respectively.31 As shown in Figure S4, three peaks can be separated from the broad peak in the N 1s spectrum. The fitted peaks centred at 398.4, 399.6, and 400.4 eV can be ascribed to the pyridinic-like (N-sp2C), pyrrolic like (N-sp3C) and graphitic nitrogen, respectively.37 The weak peak at 404.4 eV is related to the charging effects. Photocatalytic H2 production performance of as-prepared g-C3N4/RGO/MoS2 photocatalysts was evaluated under under visible light irradiation (λ > 420 nm) using TEOA as a sacrificial electron donor. Control experiments suggested that no appreciable H2 was observed in the absence of any component of photocatalyst, irradiation and TEOA, indicating that the H2 gas was evolved by light-catalyzed reaction on the photocatalyst. As shown in Figure 3a, for bare g-C3N4, a low H2 evolution rate of 13 µmol h-1 g-1 was detected owing to the rapid recombination of


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Figure 3. (a) The rate of H2 production over bare g-C3N4, g-C3N4/RGO (0.5% RGO), 1.2% gC3N4/MoS2 (1.2% MoS2) and g-C3N4/RGO/MoS2 composites loaded with 0.5% RGO and various amount of MoS2 under visible light irradiation (λ > 420 nm) in 250 mL of 0.1 M TEOA aqueous solution. (b) The rate of H2 production over g-C3N4/RGO/MoS2 photocatalysts loaded with 1.2% MoS2 and various amount of RGO under visible light irradiation (λ > 420 nm) in 250 mL of 0.1 M TEOA aqueous solution.

charge. In the presence of 0.5 wt% RGO, the photocatalytic performance of g-C3N4/RGO was slightly improved to 48 µmol h-1 g-1, perhaps because the fact that the RGO can accelerate the charge transfer rate. Even with a small content of MoS2 of 0.4 %, the H2 rate was noticeably improved to 138 µmol h-1 g-1. When the amount of MoS2 was increased to 1.2 %, the CGM (1.2) shows the maximum H2 evolution rate of 317 µmol h-1 g-1 with an apparent quantum yield of 3.4%


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at 420 nm. The photocatalytic performance of CGM (1.2) sample exceeds those of both bare gC3N4 (13 µmol h-1 g-1) and 1.2 % MoS2/g-C3N4 (98 µmol h-1 g-1) by a factor of 24.4 and 3.2, respectively. The high photocatalytic activity of CGM (1.2) sample can be assigned to the synergistic effect of RGO and MoS2. In this ternary 2D nanojunction photocatalysts, the RGO and MoS2 were used as a charge transfer “highway” and H2 evolution reaction catalyst, respectively. The photogenerated electrons can be rapidly transferred to MoS2 through RGO, resulting in the efficient charge separation for H2 evolution. However, a further increase in the amount of MoS2 results in a decreasing H2 evolution rate, which can be ascribed to the shading effect of RGO/MoS2 cocatalyst. That is, the presence of a relatively large amount of the black RGO/MoS2 cocatalyst on the surface of the g-C3N4 shielded the incident light from irradiating into the inside of the g-C3N4 light absorber, resulting in a decreased H2 evolution rate. A similar observation has been encountered in previous studies.[35,38] More importantly, the H2 evolution rate of CGM (1.2) is significantly higher than that of optimized g-C3N4/Pt photocatalyst (268 µmol h-1 g-1), as discussed later. The effect of graphene amount on photocatalytic activity of gC3N4/RGO/MoS2 was also investigated. Figure 3b shows the photocatalytic performance of gC3N4/RGO/MoS2 composites loaded with 1.2% MoS2 and different amounts of RGO under visible light irradiation. As the RGO content increases, the H2 evolution rate of gC3N4/RGO/MoS2 composites becomes higher and up to 317 µmol h-1 g-1 when the amount of RGO is 0.5%; however, a further increase in the RGO content in the g-C3N4/RGO/MoS2 composites led to a rapid deterioration of the photocatalytic activity. As illustrated in Table S2, although the g-C3N4/graphene/MoS2 composite photocatalyst exhibits a lower photocatalytic activity than those of previously-reported Pt loaded g-C3N4 nanosheets composite photocatalyst (1365 µmol h-1 g-1)39 and Ni2P-CdS co-modified g-C3N4 composite (44450 µmol h-1 g-1),40 its


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Figure 4. Cyclic H2 generation curves over g-C3N4/RGO/MoS2 [CGM1.2] photocatalyst under visible light irradiation (λ > 420 nm) in 250 mL of 0.1 M TEOA aqueous solution.

activity is much higher than those of noble-metal-free g-C3N4-based composite photocatalysts such as Ni2P/g-C3N4 (162 µmol h-1 g-1),41 Co2P/g-C3N4 (230 µmol h-1 g-1),42 Co-Pi/g-C3N4 (194.8 µmol h-1 g-1),43 and MoS2/g-C3N4 (231 µmol h-1 g-1).44 In order to investigate the durability of gC3N4/RGO/MoS2 photocatalysts, cyclic H2 evolution reactions for three times every 4 h were performed. Figure 4 shows the durability of g-C3N4/RGO/MoS2 photocatalyst under visible light irradiation. As shown in the figure, the H2 evolution rate remains virtually unchanged after 12 h of irradiation, and the H2 evolution rate during the 1st, 2nd and 3rd run was found to be 317, 314 and 313µmol h-1 g-1, respectively. After 12 h of visible light irradiation, the CGM(1.2) sample was collected from reaction solution and then characterized by XRD and TEM analysis. As shown in Figure S5, the CGM(1.2) sample exhibits similar XRD pattern as that of fresh prepared sample, indicating the structure of photocatalyst remains unchanged after photocatalytic reaction. The TEM images of CGM(1.2) sample showed that the photocatalyst has a 2D structure (Figure S6(a)) and the lattice fringe of MoS2 (d(002) = 0.61 nm) and graphene (d(001) = 0.34 nm) can still be clearly observed (Figure S6(b)). These above results suggest that the g-C3N4/RGO/MoS2 photocatalyst has a good stability during the photocatalytic H2 production reaction.


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Figure 5. (a) The rate of H2 production over g-C3N4/Pt photocatalysts under visible light irradiation (λ > 420 nm) in 250 mL of 0.1 M TEOA aqueous solution. (b) TEM of Pt-loaded gC3N4 nanosheets photocatalyst, the inset shows the magnified HRTEM image of the selected frame from TEM image. (c, d) Schematic diagrams of g-C3N4/RGO/MoS2 ternary 2D nanojunction photocatalysts (c) and 2D-0D g-C3N4/Pt photocatalysts (d), which clearly show that the g-C3N4/RGO/MoS2 2D nanojunction photocatalysts exhibit much larger contact area for interfacial charge transfer in comparison to the 2D-0D g-C3N4/Pt photocatalysts.

It has been known that Pt has a lower overpotential for H2 evolution reaction than that of graphene/MoS2 composite.45 However, in this study, the RGO/MoS2 can act as a more efficient cocatalyst than Pt metal. To reveal the possible reasons, the Pt-loaded g-C3N4 photocatalyst was separated from reaction solution and then characterized by TEM. The typical TEM image of the


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g-C3N4/Pt photocatalyst illustrated in Figure 5b shows that Pt nanoparticles with a particle size of particle 3-5 nm were loaded on the surface of g-C3N4. Furthermore, visible lattice fringes with spacing of ca. 0.23 nm was observed in the HRTEM image of g-C3N4/Pt illustrated in the inset of Figure 5b, which can be ascribed to the (111) plane of Pt.46 Based on the TEM results, it can be concluded that the g-C3N4/RGO/MoS2 and g-C3N4/Pt photocatalyst exhibits a typical 2D-2D-2D and 2D-0D structure, respectively. The typical structure schematic diagrams of 2D-2D-2D gC3N4/RGO/MoS2 and 2D-0D g-C3N4/Pt photocatalysts were illustrated in Figure 5c and d. It is clearly that the contact method exhibited in the g-C3N4/RGO/MoS2 and g-C3N4/Pt photocatalysts was face contact and point contact, respectively. The face contact provides much larger contact interfaces than that of point contact, resulting in the much higher photocatalytic activity of ternary g-C3N4/RGO/MoS2 2D nanojunction photocatalysts. Instead, the small contact area of point contact between g-C3N4 and Pt nanoparticles results in the limited interfacial charge transfer and lower photocatalytic H2 evolution activity. To gain more insight into the charge separation of as-prepared g-C3N4/RGO/MoS2 composite photocatalysts, the photoluminescence (PL) emission spectra and time-resolved emission spectra were employed to evaluate the separation efficiency of the interfacial charge transfer from gC3N4 to RGO/MoS2 since the PL emission is related to the recombination of electron-hole pairs. Figure 6a gives the comparison of PL emission spectra of bare g-C3N4, g-C3N4/RGO and gC3N4/RGO/MoS2 composites excited at 350 nm. A strong emission peak centered at 466 nm can be ascribed to the band gap transition of g-C3N4, which is in accord with with previous study.43 It is noted that the intensity of emission peak decreases after the introduction of RGO/MoS2, which indicates the efficient separation of photogenerated electron-hole pairs and a rapid charge carrier transfer in the g-C3N4/RGO/MoS2 composites. Furthermore, time-resolved emission spectra were


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Figure 6. (a) Photoluminescence spectra of bare g-C3N4, g-C3N4/RGO (0.5% RGO) and gC3N4/RGO/MoS2 composites. (b) Time-resolved photoluminescence spectra of bare g-C3N4, gC3N4/RGO (0.5% RGO) and g-C3N4/RGO/MoS2 composites. (c) Transient photocurrent responses of bare g-C3N4, g-C3N4/RGO (0.5 % RGO), g-C3N4/MoS2 (1.2 % MoS2) and gC3N4/RGO/MoS2 [CGM(1.2)] composite. (d) Electrochemical impedance spectroscopy data for bare g-C3N4, g-C3N4/RGO (0.5 % RGO), g-C3N4/MoS2 (1.2 % MoS2) and g-C3N4/RGO/MoS2 [CGM(1.2)] composite.

used to investigate the lifetimes of photogenerated charge transfer processes. As illustrated in Figure 6b, the g-C3N4, g-C3N4/RGO, CGM(0.4), CGM(0.8), CGM(1.2), CGM(1.6) and CGM(2.0) samples exhibit the average radiative lifetimes of approximately 2.1, 2.01, 1.91, 1.83, 1.77, 1.73 and 1.70 ns, respectively. The dramatically reduced exciton lifetime is related to the


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efficient interfacial charge transfer from excited g-C3N4 to RGO/MoS2, indicating the positive effect of RGO/MoS2 in accelerating charge transfer in the g-C3N4/RGO/MoS2 photocatalysts. Significantly, both fluorescence intensity and exciton lifetimes of g-C3N4/RGO/MoS2 photocatalysts decrease with the increasing amount of MoS2, which could be attributed to the fact that the MoS2 can act as an efficient electron acceptor to accelerate charge transfer. To provide an additional evidence for the improved charge carrier transfer of gC3N4/RGO/MoS2 composites, the transient photocurrent responses of bare g-C3N4, g-C3N4/RGO, g-C3N4/MoS2 and g-C3N4/RGO/MoS2 samples were investigated by repeated on-off illumination cycles. Theoretically, the higher photocurrent indicates the more efficient photogenerated charge separation and transfer. As shown in Figure 6c, the transient photocurrents of bare g-C3N4 (0.21 µA·cm-2), g-C3N4/RGO (0.62 µA·cm-2), g-C3N4/MoS2 (0.69 µA·cm-2) and g-C3N4/RGO/MoS2 (1.31 µA·cm-2) showed that the samples loaded with a cocatalyst (RGO, MoS2 or RGO/MoS2) exhibit much higher photocurrents than that of pure g-C3N4, suggesting the importance of cocatalyst in the process of charge separation and transfer. The g-C3N4/RGO/MoS2 sample shows the highest photocurrent, which probably resulted from the synergistic effect of RGO and MoS2. That is, the RGO and MoS2 can act as an electron transfer channel and electron sink, respectively, resulting in the more efficient separation of photogenerated charge carrier. Furthermore, the electrochemical impedance spectra (EIS) analysis was used to further evaluate the charge transfer property. The EIS results illustrated in Figure 6d show that the gC3N4/RGO/MoS2 sample exhibits the smallest resistance for charge transfer, which benefits from the superior charge separation of photogenerated charge carriers. Combining those above-mentioned photophysical and electrochemical studies, a tentative mechanism for the enhanced photocatalytic H2 production performance of g-C3N4/RGO/MoS2


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photocatalysts is proposed. As illustrated in Figure S7, the electrons located at the valance band (VB) of g-C3N4 can be excited to the conduction band (CB) under visible light irradiation, thereby creating the electron-hole pairs. The CB of g-C3N4 [Ecb(g-C3N4) = -1.3 V vs. NHE, pH 7] is more positive than the graphene/graphene·- redox potential [E (G/G·-) = -0.57 V vs. NHE, pH 7], thus providing sufficient driving force for electron transfer from CB of g-C3N4 to RGO.47,48 The photogenerated electrons can be further transferred to MoS2 because the graphene/graphene·redox potential is more positive than that of MoS2 nanosheets [Ecb(MoS2) = -0.54 V vs. NHE, pH 7].30 The charge transfer between g-C3N4 and RGO/MoS2 cocatalyst was demonstrated by photoluminescence and transient photocurrent studies. The crystallite edges of MoS2 can act as the reaction active sites for H2 generation after they accepted electrons from electron-rich RGO.49,50 Furthermore, the VB of g-C3N4 [EVB(g-C3N4) = +1.4 V vs. NHE, pH 7]46 is much higher than the oxidation potential of TEOA [E(TEOA+/TEOA) = +0.82 V vs. NHE, pH 7],51 thus the oxidized g-C3N4 can be reduced by TEOA to regenerate ground-state g-C3N4. During the photocatalytic reaction processes, the intimate 2D nanojunction exhibited in the gC3N4/RGO/MoS2 photocatalyst can act as an electron transport bridge to accelerate the charge transfer from g-C3N4 to MoS2, thus resulting in the enhanced photocatalytic H2 production performance of g-C3N4/RGO/MoS2 photocatalyst. In addition, some photogenerated electrons could also be transferred to the MoS2 on the surface of g-C3N4 and then react with protons to produce H2. In summary, we have successfully constructed unique ternary g-C3N4/RGO/MoS2 2D nanojunction photocatalysts for visible light photocatalytic H2 production from water. The gC3N4/RGO/MoS2 photocatalyst shows the highest H2 evolution activity with a rate as high as 317 µmol h-1 g-1 for the composite containing 0.5% RGO and 1.2% MoS2, and the apparent quantum


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yield achieves 3.4% at 420 nm. More importantly, the optimized ternary g-C3N4/RGO/MoS2 2D nanojunction photocatalyst exhibits much higher photocatalytic activity than the optimized Ptloaded g-C3N4 photocatalyst. It is believed that the intimate two-dimensional nanojuctions exhibited in g-C3N4/RGO/MoS2 photocatalysts can efficiently suppress charge recombination and accelerate the charge transfer, resulting in the higher photocatalytic activity. The enhanced charge transfer was demonstrated by photoluminescence and transient photocurrent studies. This study shows that the construction of two-dimensional nanojunction in composite photocatalyst is an efficient strategy to improve charge separation efficiency and photocatalytic activity. Supporting Information. Experimental details, N2 adsorption-desorption isotherms and the pore size distribution plots, high resolution XPS spectrum of N 1s and C1s, XRD pattern, TEM and HRTEM images, energy band diagram of g-C3N4/RGO/MoS2 photocatalysts, comparison of photocatalytic performance for H2 production between the current work and other reported studies. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] ORCID Yong-Jun Yuan: 0000-0002-1823-3174; Daqin Chen: 0000-0003-0088-2480. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This research was supported by the National Natural Science Foundation of China under Grant No. 51772071, 51502068, 61504063 and 51572065, the Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ16B030002, the Natural Science Foundation of Jiangsu Higher Education Institutions of China under Grant No. 15KJB430023, the Zhejiang Provincial Natural Science Foundation for Distinguished Young Scholars under Grant No. LR15E020001. REFERENCES (1) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Mmaterials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (2) Ma, Y.; Wang, X. L.; Jia, Y. S.; Chen, X. B.; Han, H. X.; Li, C. Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations. Chem. Rev. 2014, 114, 9987-10043. (3) Yuan, Y. J.; Yu, Z. T.; Chen, D. Q.; Zou, Z. G. Metal-complex Chromophores for Solar Hydrogen Generation. Chem. Soc. Rev. 2017, 46, 603-631. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (5) Yuan, Y. J.; Ye, Z. J.; Lu, H. W.; Hu, B.; Li, Y. H.; Chen, D. Q.; Zhong, J. S.; Yu, Z. T.; Zou, Z. G. Constructing Anatase TiO2 Nanosheets with Exposed (001) Facets/Layered MoS2 Two-Dimensional Nanojunctions for Enhanced Solar Hydrogen Generation. ACS Catal. 2016, 6, 532-541. (6) Lin, Z. Y.; Xiao, J.; Li, L. H.; Liu, P.; Wang, C. X.; Yang, G. W. NanodiamondEmbedded p-Type Copper(I) Oxide Nanocrystals for Broad-Spectrum Photocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 1501865. (7) Han, K.; Kreuger, T.; Mei, B.; Mul, G. Transient Behavior of Ni@NiOx Functionalized SrTiO3 in Overall Water Splitting. ACS Catal. 2017, 7, 1610-1614. (8) Yuan, Y. J.; Chen, D. Q.; Yang, S. H.; Yang, L. X.; Wang, J. J.; Cao, D. P.; Tu, W. G.; Yu, Z. T.; Zou, Z. G. Constructing Noble-Metal-Free Z-Scheme Photocatalytic Overall Water Splitting Systems using MoS2 Nanosheet Modified CdS as a H2 Evolution Photocatalyst. J. Mater. Chem. A 2017, 5, 21205-21213. (9) Ma, S.; Xie, J.; Wen, J. Q.; He, K. L.; Li, X.; Liu, W.; Zhang, X. C. Constructing 2D Layered Hybrid CdS Nanosheets/MoS2 Heterojunctions. Appl. Surf. Sci. 2017, 391,


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