GO Ternary Heterojunction with

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Facile Synthesis of MoS2/g-C3N4/GO Ternary Heterojunction with Enhanced Photocatalytic Activity for Water Splitting Min Wang, Peng Ju, Jiajia Li, Yun Zhao, ## #, and Zhaomin Hao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01386 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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ACS Sustainable Chemistry & Engineering

Facile Synthesis of MoS2/g-C3N4/GO Ternary Heterojunction with Enhanced Photocatalytic Activity for Water Splitting Min Wang,a, b Peng Ju,a Jiajia Li,a, b Yun Zhao,a, b Xiuxun Han a,c * and Zhaomin Hao d a

Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, PR China b

University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, PR China

c

Qingdao Center of Resource Chemistry & New Materials, 36 Jinshui Road, Qingdao 266100, PR

China d

Henan University, 85 Minglun Road, Kaifeng 410200, PR China

*Corresponding author. Tel.: +86-931-4968054; Fax: +86-931-4968054 E-mail: [email protected] (X.X. Han) Abstract Basing on a simple ion exchange method, a MoS2/g-C3N4/graphene oxide (GO) ternary nanojunction was constructed as an efficient photocatalyst for hydrogen evolution using solar energy. The confinement effect in MoS2 and g-C3N4 quantum dots enhances their redox activities in water splitting. The designed heterostructure possesses featured band alignment that facilitates the collection of electrons in MoS2 and holes in g-C3N4, effectively suppressing the recombination of photogenerated charge carriers. Furthermore, the GO with high specific surface area serves as an excellent conductive substrate to transport holes speedily. This study thus provides a novel and facile route of establishing efficient composite photocatalyst with multinary components for energy conversion. Key words MoS2, C3N4, GO, heterojunction, water splitting

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Introduction Since Honda and Fujishima reported the hydrogen production by photoelectrocatalysis method using TiO2 in 1972, considerable studies have been focused on seeking advanced materials that could act as both the light absorber and energy converter to store solar energy in the hydrogen bond.1-3 In principle, a heterogeneous photocatalytic reaction involves the generation, migration, and separation of charge carriers in the light-excited semiconductors, as well as the redox reactions with the separated electrons and holes at a semiconductor/aqueous interface.4,5 To achieve these aims, many researches have paid attentions to perfect the intrinsic physicochemical properties of semiconductors, including the band gap, direct/indirect transition, band position, particle size, surface area, and morphology.6-9 However, the efficiency is far from satisfactory due to the recombination of charge carriers in the photocatalytic reactions, which restricts the photocatalytic activity of semiconductors seriously. Hence, it is in urgent need of promoting the separation of charge carriers to further enhance the photocatalytic performance in photocatalysis. Constructing heterostructure between semiconductors is one of promising strategies to improve photocatalytic activity.5,10-13 Depending on the relative band position of those two semiconductors, the formed heterojunction can be classified into three different types, among which, heterojunctions with staggered band alignment (type II) is the optimum structure to promote the efficient charge carrier separation.14-19 Recently emerged studies on ternary junctions such as Ag2CO3/Ag/AgBr,20 CdS/Au/ZnO,21 and CNNS (g-C3N4 nanosheets)/NRGO (nitrogen-doped graphene)/MoS222 have gained increasing research attention. In CNNS/NRGO/MoS2 system, layered MoS2 combined with the CNNS not only enhances the light absorption to generate more photoelectrons but also promotes charge separation at CNNS/MoS2 interfaces (sheet to sheet) owing to the type II alignment, while NRGO

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interlayers work as the mediator for shuttling electrons-holes between CNNS and MoS2 sheets.23 As a result, it exhibits an efficient catalytic activity in solar conversion. However, the simultaneously direct contact of NRGO with both CNNS and MoS2 might also lead to the unwanted recombination of photogenerated carriers. In fact, the speedy transfer of carriers is especially critical in photocatalysts at the nanoscale after their effective separation at the interface. Hence, combining a heterojunction onto noble metals (such as Ag, Au) or graphene with exclusive contact of the designated component can be expected to improve photocatalytic behavior further, but remains as a significant challenge for practical construction. In this study, we report a facile method to construct a ternary MoS2/g-C3N4/GO nanojunction consisting of GO thin films, g-C3N4 quantum dots (CNQDs), and MoS2 QDs. By reasonably optimizing the location of MoS2, C3N4 and GO, both of GO and MoS2 can only contact with C3N4. In the prepared composite, a type-II heterostructure is formed at the interface between g-C3N4 and MoS2, promoting the effective migration of photogenerated electron-hole pairs. Moreover, GO mainly acts as the hole transport layer, achieving the efficient transfer of collected carriers in g-C3N4. Compared with 2D CNNS that tends to fold, CNQDs are proposed with the conjunct effects: 1) the improved interfacial contact between g-C3N4 and GO; 2) the facilitated transfer of photogenerated holes from g-C3N4 to GO acceptor. The as-prepared MoS2/g-C3N4/GO ternary heterojunctions are investigated in terms of morphology, composition, crystal structure, photocurrent, and photocatalytic response, and exhibit significantly enhanced photoelectrocatalytic and photocatalytic activities. Experimental Section Synthesis of photocatalysts The graphene oxide (GO) was synthesized through chemical exfoliation of graphite powders

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according to the Hummers’ method.24 For a typical synthesis of GO, 1.0 g of graphite powder was dispersed into 23 mL concentrated H2SO4 under ice bath. 3.0 g KMnO4 was added gradually under vigorous stirring. The mixture was ultrasonicated at room temperature for 10 h until thoroughly dispersed. Then 46 mL H2O and 150 mL 9% H2O2 solution were added, respectively. After standing for 5 h, the precipitate was filtered and washed with 0.2 M HCl solution. MoS2 QDs were prepared according to a reported method.25 In short, 100 mg of MoS2 powder (Sigma-Aldrich) was added into 10 mL of 1-methyl-2-pyrrolidone (NMP) in a 20 mL beaker and sonicated in an ultrasonic bath continuously for 3.5 h. Then the dispersion was kept undisturbed overnight, and the top suspension was centrifuged at 5500 r min-1 for 90 min. Finally, a green product was obtained. To prepare bulk g-C3N4 (BCN), 10.0 g of melamine powder was put into an alumina crucible with a cover, and heated to 600 °C in a muffle furnace for 4 h with a heating rate of 2.3 °C min-1. The obtained yellow products were collected and ground into powder for further processing and characterization.9 Grinded BCN powders were further used to obtain g-C3N4 quantum dots (CNQDs).26 Typically, BCN was placed in an open ceramic container and was heated at 500 °C for 2 h with a ramp rate of 2 °C min-1 to get g-C3N4 nanosheets. Then, g-C3N4 nanosheets (0.05 g) were oxidized in concentrated H2SO4 (10 mL) and HNO3 (30 mL) for 16 h under mild ultrasonication. A clear solution was formed, which was then diluted with deionized water (200 mL) to produce a colloidal suspension. The suspension was transferred to a Teflon-lined stainless steel autoclave and heated at 200 °C for 10 h. After cooling to room temperature, the final product of yellowish CNQDs solution was obtained. MoS2/g-C3N4/GO composite was synthesized by two steps. Firstly, CNQDs/GO was prepared by

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stirring 2 mL CNQDs (0.14 g L-1) with 2.0 mL GO (5.0 g L-1) for 3 h at room temperature. The obtained samples were labeled as 2CNQDs/GO, where 2 refer to the volume of CNQDs. Subsequently, 3CNQDs/GO, 4CNQDs/GO, and 5CNQDs/GO samples were obtained via the same process with 3, 4, and 5 mL CNQDs (0.14 g L-1) added, respectively. Then, MoS2/g-C3N4/GO composite was synthesized via an anion-exchange and hydrothermal approach. In a typical process, 72 mL CNQDs/GO and 0.66 g of Na2MoO4·2H2O were added into a 100 mL flask successively, and then stirred at room temperature for 48 h. After centrifugal washing 9 times at 15000 r min-1, 0.1 g TAA (CH3CSNH2) was added and stirred for another 1 h. The resultant suspension were sealed in a Teflon-lined stainless steel autoclave and heated at 180 °C for 18 h. Thus, the MoS2/g-C3N4/GO heterostructure was obtained. Accordingly, MoS2/g-C3N4/GO composites were labeled as MoS2/xg-C3N4/GO, where x refers to the volume of CNQDs introduced in the first step. Furthermore, MoS2/g-C3N4 composite was synthesized via a parallel process without the introduction of GO. MoS2/GO was also synthesized via a parallel process without the introduction of g-C3N4. Characterization The transmission electron microscope (TEM) images, energy dispersive X-ray spectroscopy (EDX), and elemental mapping of the as-prepared samples were examined on a JEOL JEM-2100 coupled with an EDX detector. A Physical Electronics PHI-5702 spectrometer was applied to record the X-ray photoelectron spectrum (XPS) with an Mg Kα X-ray source (250 W, pass energy of 29.35 eV). All of the binding energies were calibrated by the C 1s peak at 284.6 eV. The X-ray diffraction (XRD) patterns were acquired on a Germany Bruker D8 Advanced powder diffractometer using Cu Kα radiation (λ = 0.15406 nm). UV-vis absorption spectra of the samples were recorded on a Shimadzu UV-3600 apparatus. The structural information for the samples was measured by Fourier transform

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spectrophotometer (FT-IR, Brucker Tensor 27) with KBr as the reference sample. The zeta potentials of 0.5 mg mL-1 particle suspensions were obtained from a Zeta PALS instrument (Brookhaven Instruments Co., New York). Contents of S and Mo in MoS2/g-C3N4/GO samples were determined by a 725-ES inductively coupled plasma emission spectrometer (ICP-AES). Atomic Force Microscope (AFM) image was acquired using a Digital Instrument Shimadzu SPM-9600 (The sample was prepared by drop-casting corresponding dilute dispersion onto freshly cleaved mica surface). Photoelectrochemical analysis Photoelectrochemical measurements were conducted in a conventional three-electrode cell system by using a CHI 660E electrochemical station with a 300 W Xenon lamp as light source. The ITO glass deposited with samples, a Pt wire and an Ag/AgCl electrode (3.0 M KCl) were used as working electrodes, counter electrode and reference electrode, respectively. Nyquist plot measurement was done under a perturbation signal of 20 mV, and the frequency ranged from 10-2 to 105 Hz. A 0.5 M H2SO4 (pH = 0) aqueous solution without additive was utilized as the electrolyte and purged with nitrogen gas for 30 min prior to the measurements. For photocurrent tests, the electrolyte was an aqueous solution of 0.1 M KCl/0.1 M Eu(NO3)3, and current density-time (J-t) curves were measured under an applied potential of 0.4 V versus RHE. Photocatalytic test Photocatalytic reactions were carried out in a quartz top-irradiation reaction vessel. A 450 W Xenon lamp coupled with an AM 1.5G filter (Solar simulator, Newport, 94023A) was applied as the simulated sunlight source (I0 = 100 mW cm-2). H2 production was performed by dispersing 4.0 mg of catalyst in an aqueous solution (20 mL) containing Na2SO3 (0.25 M). The reactant solution was degassed several times to remove air prior to irradiation, and the produced gas was analyzed by gas chromatography

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equipped with a thermal conductive detector and high-purity Ar as the carrier gas. For stability test, MoS2/4g-C3N4/GO catalyst was withdrawn at regular intervals (5 h). Results and discussion TEM analysis

Figure 1 TEM images of (a) GO, (b) CNQDs, (c) 4CNQDs/GO, and (d) MoS2/4g-C3N4/GO; HRTEM image (e) and SAED pattern (f) of MoS2/4g-C3N4/GO; The corresponding elemental mapping images of C (g), N (h), Mo (i), and S (j) in MoS2/4g-C3N4/GO. Inset: The size distribution of the as-prepared CNQDs evaluated from the TEM image by measuring about 100 individual dots. The morphologies and microstructures of GO, CNQDs, 4CNQDs/GO, and MoS2/4g-C3N4/GO are shown in Figure 1. It can be seen that GO displays a typical morphology of a layered structure with wrinkles (Figure 1a). Also, the nearly transparent feature of the nanofilms indicates its ultrathin thickness. Besides, as clearly illustrated in Figure 1b, uniform CNQDs are observed after the sonication treatment, and the diameters of the CNQDs, extracted by directly measuring 100 particles from the TEM analysis, are in the range of 5-9 nm (average diameter is estimated to be approximately 6.7 nm) with slight aggregation. When the CNQDs are deposited onto the layered GO via the electrostatic bond, however, the dispersity of CNQDs is improved distinctly (Figure 1c). Typical TEM image of MoS2/4g-C3N4/GO (Figure 1d) shows that rich CNQDs can be seen on the gauze-like substrate, and 7



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some of them are decorated with MoS2 QDs (dark dots). Subsequently, AFM was conducted to investigate the morphology of CNQDs and MoS2 QDs. As shown in Figure S1, the heights of CNQDs and MoS2 QDs are determined to be about 2.7 and 5.1 nm. Meanwhile, the lateral sizes of CNQDs and MoS2 QDs are determined to be about 8.3 and 5.5 nm. A more close examination (Figure 1e) reveals the pyramid structure of the ternary composite: relatively small MoS2 QDs packed closely onto CNQDs, and CNQDs packed closely on the GO layer. Meanwhile, a lattice spacing of 0.27 nm is clearly discerned, which corresponds to the (100) plane of MoS2,27 indicating that the ternary composites have been successfully formed in the hydrothermal process. Furthermore, the selected area electron diffraction (SAED) pattern of the MoS2/4g-C3N4/GO nanojunction (Figure 1f) shows obvious diffraction rings, which can be identified as MoS2 and GO,22 respectively, further implying that a well-defined MoS2/g-C3N4/GO heterostructure has been formed. Besides, the existence MoS2 of in the MoS2/g-C3N4/GO composites is further identified by the elemental mapping images (Figure 1g-j). It can be seen that the C, N, Mo, and S elements are homogeneously distributed in the GO sheet. In addition, the elemental analyses via the EDX line scan (Figure S2) are well consistent with the observed construction of MoS2/g-C3N4/GO heterojunction from the TEM, HRTEM image and the SAED pattern. XRD analysis

Figure 2 (a) XRD patterns of MoS2 QDs, CNQDs, GO, and MoS2/4g-C3N4/GO. (b) XPS survey spectra of MoS2/4g-C3N4/GO. High resolution XPS spectra of (c) C 1s, (d) N1s, (e) Mo 3d, and (f) S 2p

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for MoS2/4g-C3N4/GO.

The crystal structure of the obtained samples was analyzed by X-ray diffraction (XRD). Figure 2a shows the weak (100) and (110) peaks of hexagonal MoS2 (JCPDS Card No. 37-1492) in pure MoS2 due to the size effect of quantum dots. Meanwhile, the weak (002) peak of CNQDs (JCPDS Card No. 87-1526) at 29.5° indicates the nanoscale size of g-C3N4. In addition, the sharp (002) peak of GO at 14.9° indicates the good crystallinity of GO. In MoS2/4g-C3N4/GO composites, all diffraction peaks can be indexed to GO, CNNS and MoS2. These results are in good agreement with HRTEM and SAED analysis. To gain more information of the structure of MoS2/g-C3N4/GO composites, we conducted XPS analyses to further study the valence state of this photocatalysts. The XPS survey spectra (Figure 2b) suggest that the main chemical components of MoS2/g-C3N4/GO composites are carbon, nitrogen, molybdenum, and sulfur, identifying the existence of MoS2 in the composite. The high-resolution C 1s spectrum (Figure 2c) of MoS2/g-C3N4/GO displays five fitted peaks at 284.6, 285.4, 286.6, 288.3, and 289.2 eV, which are assigned to the C-C, C(O)O and C=N, C=C, C-N, and C-OH configurations, respectively, implying there are GO and C3N4 in the MoS2/g-C3N4/GO composite.22 The corresponding N 1s spectrum (Figure 2d) can be deconvoluted into three peaks, which are attributed to sp3 terminal N (399.7 eV), tertiary N (398.4 eV), and aromatic N (397.8 eV), respectively. This is consistent with the observation from g-C3N4.28 Besides, the Mo 3d spectra (Figure 2e) show peaks at around 228.9 and 232.2 eV, corresponding to Mo4+ 3d5/2 and Mo4+ 3d3/2 of MoS2, respectively. Figure 2f clearly depicts the S-related peak, S2- 2p1/2 at 161.7 eV and S2- 2p3/2 at 162.8 eV, which are consistent with the reported values of MoS2 crystals.31 Formation mechanism

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Scheme 1 Schematic illustration of the proposed formation mechanism for the MoS2/g-C3N4/GO composite. Based on the above results, the formation mechanism of the MoS2/g-C3N4/GO composite is proposed, as illustrated in Scheme 1. There are abundant oxygenated functional groups on the surface of GO derived from the oxidation process (Figure S3), leading to a negative zeta potential on the surface of GO (Figure S4). Besides, with the assistance of the protonation due to acidic cutting, CNQDs exhibit positive potentials (Figure S5). Resulting from the pre-treatment, GO and CNQDs thus possess negative and positive surface charge respectively. Therefore, CNQDs/GO junctions could be formed under the assistance of electrostatic forces by stirring GO and CNQDs mixture. An ion exchange strategy was subsequently applied to guarantee the in situ nucleation and growth of MoS2 QDs just on the CNQDs. After the introduction of Na2MoO4·2H2O, MoO42- can be electronically absorbed onto the positively charged surface of CNQDs, while absent from the negatively charged surface of GO owing to the charge repulsion. In the final step, CH3CSNH2 (TAA) is added into the mixture to supply the sulfur during the hydrothermal reaction, and MoS2/g-C3N4/GO composites with well-defined heterostructure could be obtained. To further explore this formation mechanism, 10


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MoS2/GO was also synthesized via a parallel process without the introduction of g-C3N4. The TEM image of MoS2/GO displays that no MoS2 particles can be observed on GO films (Figure S6a). Besides, only C element is detected in EDX (Figure S6b) and XPS (Figure S6c), elucidating that MoS2 cannot directly deposit on GO films due to the rich negative functional groups on the basal plane and at the edges of GO films. In the assembly process of MoS2/g-C3N4/GO composites, GO serves as an excellent supporting matrix, which inhibits the aggregation of quantum dots. UV-vis and photocurrent analysis

Figure 3 (a) UV-vis absorption spectra of MoS2 QDs, CNQDs, GO, 4CNQDs/GO, and MoS2/4g-C3N4/GO. (b) Current density-voltage curves of MoS2 QDs, CNQDs, MoS2/g-C3N4, and MoS2/4g-C3N4/GO. (c) Transient photocurrent density of MoS2 QDs, CNQDs, 4CNQDs/GO, and MoS2/4g-C3N4/GO under intermittent light illumination. (d) Photocurrent responses of the MoS2/4g-C3N4/GO composite under light irradiation at 0 V vs. RHE. The optical properties of the samples were investigated by UV-vis absorption spectroscopy (Figure 3a). It can be seen that the CNQDs exhibit intense absorbance in the ultraviolet region (λ < 350 11



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nm) owing to the quantum confinement.8 The loading of CNQDs onto GO via electrostatic forces does not change the absorption peak significantly except for the relatively enhanced intensity over the entire range of wavelengths.23 Four characteristic absorption bands are observed in MoS2 QDs, which is a typical behavior of MoS2 QDs.18 Intense light absorption finally extends into the visible-light region for MoS2/g-C3N4/GO composite after loading MoS2 QDs owing to its relatively narrower band gap. The stronger background absorption that inherits from the GO is also evident.29 The photoelectrochemical (PEC) performances of the as-prepared catalysts were measured with an Ag/AgCl reference electrode and a Pt wire counter electrode. Figure 3b shows typical current density-potential curves obtained under chopped illumination. We observe that the onset of hydrogen evaluation reaction (HER) is significantly shifted to lower potential value for MoS2/4g-C3N4/GO (43 mV) in comparison to the other catalysts (e.g. 102 mV for MoS2/g-C3N4, 183 mV for CNQDs, 286 mV for MoS2 QDs). In addition, pure CNQDs display very low photocurrent resulting from its large band gap (2.83 eV) and the rapid recombination of photoinduced carriers. Meanwhile, a higher photocurrent is observed in MoS2 QDs because of its appropriate band gap (1.82 eV). Comparably, MoS2/g-C3N4 electrode presents the increasing photocurrent due to the enhancement in the light absorption ability (Figure 3a), and the appreciable improved photocurrent is also owing to the enhancement of spatial charge separation due to the formation of a type-II heterojunction structure at the interface between g-C3N4 and MoS2. Furthermore, the photocurrent is dramatically increased by incorporating GO. This enhancement should be caused by the reduced carrier recombination loss due to the hole transport layer. These results suggest that the enhanced light harvesting (Figure 3a), lower onset potential value (Figure 3b), and efficient separation of photogenerated electron-hole pairs play important roles in improving the catalytic performance. Therefore, constructing a type-II heterojunction and incorporating a hole

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transport layer are beneficial for PEC activity enhancement of MoS2, and an improved photocatalytic performance can be expected. Besides, the transient photocurrents of MoS2 QDs, CNQDs, CNQDs/GO, and MoS2/4g-C3N4/GO were characterized during repeated ON/OFF illumination cycles. Here, 0.4 V (vs. RHE) was chosen as reference potential for the evaluations of current density, considering the fact that all the investigated systems were under HER conditions and showed noticeable photocurrent density at this potential value. It can be seen in Figure 3c that all samples exhibit prompt photocurrent responses to each illumination because of the emergence of photogenerated charge carriers. When the irradiation is interrupted, the photocurrent will drop rapidly to almost zero owing to the anodic current driven by minority carrier.30 Besides, it can be seen that the transient photocurrent density of the CNQDs/GO is higher than that of pure CNQDs, implying the positive effect of GO in promoting carrier shuttling, where GO can act as a carrier transfer channel, facilitating the photogenerated carrier transfer from CNQDs. Moreover, the MoS2/4g-C3N4/GO composite shows a much higher photocurrent than those of MoS2 QDs, CNQDs and CNQDs/GO, indicating the effective separation of photogenerated charge carriers in the heterojunction structure. Furthermore, these results demonstrate that the composite is capable of harvesting light in a wider spectrum region than the individual components to generate more electron-hole pairs, which are in good consistent with the results from current density-potential curves. Another interesting finding is that no significant photocurrent change is observed within 6000 s of illumination for the MoS2/g-C3N4/GO composites, indicating a nice stability of the heterostructures (Figure 3d). The MoS2/g-C3N4/GO composites show high stability because its photoinduced holes (h+) can migrate to GO side efficiently through the constructed heterojunction structure. With the fast transfer of photogenerated charges, a low concentration of h+ was left behind in MoS2. Hence, MoS2 in the

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composite can avoid photo-corrosion of sulfur during solar water splitting reaction. The elemental and XPS analysis on the fresh and used MoS2/4g-C3N4/GO was performed and compared to illustrate the stability of the composite. It can be seen in Table S1 that no significant changes can be observed in the content of S within 6000 s illumination for the MoS2/4g-C3N4/GO composites. Meanwhile, Figure S7 shows the XPS spectra of S 2p, indicating that the spectra are almost the same for the fresh and used photocatalysts, which further confirms the excellent stability of the composite. Photocatalytic activity of the catalyst

Figure 4 Photocatalytic H2 production rates of (a) CNQDs, CNQDs/GO, MoS2/4g-C3N4/GO, and MoS2 QDs, (b) MoS2/g-C3N4/GO with different loading amounts of MoS2/g-C3N4, (c) MoS2 QDs, CNQDs, MoS2/4g-C3N4/GO, physical-mixture sample of MoS2 QDs + CNQDs + GO, and the mixture with hydrothermal treatment, and (d) MoS2 QDs, MoS2/g-C3N4, and MoS2/4g-C3N4/GO heterojunction. Photocatalytic hydrogen production experiments were carried out by using aforementioned samples and Na2SO3 (as a sacrificial agent, 0.25 M) under simulated AM1.5G solar irradiation. As shown in Figure 4a, MoS2/g-C3N4/GO heterojunction displays an enhanced H2 evolution activity over

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the corresponding host substrates because of its effective separation of photogenerated charge carriers. It can be seen in Figure 4b that the MoS2/g-C3N4 content in the MoS2/g-C3N4/GO composite played a key role in achieving the high photocatalytic activity of the MoS2/g-C3N4/GO catalyst. In addition, the rate of hydrogen evolution increases initially with the rising load amount, and then reaches a maximum when the load amount of MoS2/g-C3N4 is about 5.6 wt% (Table S2). Further increasing the amount of MoS2/g-C3N4, however, reduces the photocatalytic hydrogen production rate, which can be attributed to the fact that loading more MoS2/g-C3N4 in the composite will lead to the aggregation of MoS2/g-C3N4 QDs and impact the effective contact between g-C3N4 and GO (Figure S8). To demonstrate the necessity of tight interconnection for heterojunctions, the physical-mixture sample of MoS2 QDs + CNQDs + GO and the mixture with hydrothermal process were prepared and the photocatalytic activities were tested. As demonstrated in Figure 4c, no significant enhancement in photocatalytic activity could be observed. To clearly demonstrate the role of MoS2 QDs, g-C3N4, and GO sheets in the photocatalytic

performance,

photocatalytic

activities

of

MoS2

QDs,

MoS2/g-C3N4,

and

MoS2/4g-C3N4/GO were compared, as shown in Figure 4d. It can be seen that MoS2/g-C3N4 shows significantly enhanced H2 production rate of 1.06 mmol h-1 g-1, which is about 4.3 times higher than that of MoS2 QDs (0.20 mmol h-1 g-1). This can be ascribed to the type-II heterojunction structure at the interface between g-C3N4 and MoS2, which facilities the separation of photogenerated charge carriers. Besides, MoS2/g-C3N4/GO heterostructure shows an even higher H2 production rate (1.65 mmol h-1 g-1) compared with MoS2/g-C3N4, which is associated with the fact that the separation efficiency of electrons and holes are further enhanced by the rapid charge transfer at the interface of g-C3N4 and GO. Furthermore, the stability of MoS2/g-C3N4/GO nanojunctions was examined by cycling runs for the photocatalytic H2 evolution (Figure 5a). The test results show that the photocatalytic performance

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of MoS2/g-C3N4/GO nanojunctions can be very stable for six cycles (30 h) without observation of any significant loss in activity, confirming the high stability of MoS2/g-C3N4/GO system during photocatalytic H2 production. After the circling operations, FT-IR and XPS measurements were performed on the recycled MoS2/g-C3N4/GO composite (Figure 5). It can be seen in Figure 5b that after photocatalytic reaction typical absorption bands of oxy-functional groups decrease significantly, such as the vibration of hydroxyl groups on the plane at 3410 cm-1 and the vibration of carboxyl groups situated at the edges of GO sheets at 1396 cm-1, indicating that the GO has been reduced to rGO by light irradiation. Identical conclusions were obtained in XPS analyses. The reduction of GO was verified by the absent of C-OH, C=O, and O=C-OH configurations in the high resolution XPS spectra of C 1s of used MoS2/g-C3N4/GO (Figure 5c). Besides, compared with the MoS2/4g-C3N4/GO samples before catalytic reaction, the content and valence state of N, Mo, and S in the recycled MoS2/4g-C3N4/GO photocatalysts show negligible changes (Figure 5d-f), implying the high chemical stability of g-C3N4 and MoS2. Firstly, g-C3N4 demonstrates high stability due to its inherent structure.9 Secondly, in this work MoS2 displays excellent stability resulting from the efficient transfer of its photoinduced holes (h+) to GO side through the constructed heterojunction structure, thus avoiding photo-corrosion of sulfur during solar water splitting reaction. Thirdly, GO has been reduced to rGO by light irradiation in the photocatalytic reaction, facilitating the transport of carries and enhancing the photocatalytic activity of the composite photocatalysts. Hence, MoS2/g-C3N4/GO photocatalysts present superior stability and reusability in catalytic H2 evolution reaction.

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Figure 5 (a) Cycling test of photocatalytic H2 evolution for MoS2/4g-C3N4/GO. (b) FT-IR spectrum of MoS2/4g-C3N4/GO before and after photocatalytic reaction. High resolution XPS spectra of C 1s (c), N 1s (d), Mo 3d (e), and S 2p (f) in MoS2/4g-C3N4/GO before and after photocatalytic reaction. Mechanism of the enhanced photocatalytic performance in the MoS2/g-C3N4/GO system Based on these experimental results, a model of the ternary heterostructures was proposed, as illustrated in Figure 6a. It can be seen that each island forms a nanoscale type-II junction at the MoS2/g-C3N4 interface, which plays a crucial role in the photocatalytic process. In addition, GO leads a decrease in recombination of electron-hole pairs, working as the hole transport layer. In order to study the band alignment of the MoS2/g-C3N4 junctions, the band gap of MoS2 QDs and CNQDs were estimated to be 1.82 and 2.83 eV (Figure S9), respectively.31-33 Thus the band edges of the conduction band (CB) of MoS2 QDs was determined to be -0.08 V (vs. RHE) according to the empirical equation (more positive than that of CNQDs (ECB = -1.22 V)). The valence band (VB) of MoS2 QDs was subsequently estimated to be 1.74 V (more positive than that of CNQDs (EVB = 1.61 V)).34-37 To further confirm the band structure of CNQDs and MoS2 QDs, the electrochemical Mott-Schottky plots of CNQDs and MoS2 QDs samples were determined (Figure S10), which give values of flat band potential at -1.21 and -0.10 V for CNQDs and MoS2 QDs, respectively.9,10,38-40 Together with the band

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gap energy, the CB and VB of CNQDs can be estimated to be -1.21 and +1.62 V (vs. RHE), respectively. Accordingly, the CB and VB of MoS2 QDs can be estimated to be -0.10 and +1.72 V, which coincide with the results estimated from empirical equation. Meanwhile, the work function values of GO thin film samples was found to be 0.5 V (vs. RHE),41 negative than VB of CNQDs. Therefore, once MoS2 and g-C3N4 are integrated together, the band alignment between the two materials resulting in the formation of the type-II junction as indicated in Figure 6b.42 Thus photogenerated holes will move to the CNQDs side, and electrons will move to the MoS2 QDs side. Besides, the work function value of GO can match well with the VB level of CNQDs, leading holes in CNQDs can then transfer to GO to form the charge-separated state (MoS2-/g-C3N4/GO+), competing with the recombination process of electron-hole pairs. Thanks to the unique spatial distribution of the top layer MoS2 QDs-localized electron and the bottom layer GO-localized hole in the ternary MoS2/g-C3N4/GO composite, the reasonable spatial distribution enables ultrafast charge separation while simultaneously retarding the charge recombination. Therefore, electrochemical impedance spectroscopy (ESI) is used to reveal the efficiency of charge carrier trapping, transfer, and separation of MoS2 QDs, CNQDs, CNQDs/GO, and MoS2/4g-C3N4/GO catalysts. As shown in Figure 6c, CNQDs/GO exhibits a smaller charge transfer resistance than CNQDs, which is mainly due to the decrease in recombination of electrons and holes in the CNQDs/GO electrode with GO as the hole transport layer. Besides, MoS2/g-C3N4/GO heterostructure possesses a significantly decreased diameter, proving that the MoS2/g-C3N4/GO heterojunction has the lowest electron-hole recombination rate compared with the pure MoS2 QDs and CNQDs counterparts. As a result, the MoS2/g-C3N4/GO composite demonstrates the significantly improved photocatalytic activity.

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Figure 6 (a) Schematic drawing of the MoS2/g-C3N4/GO composite, (b) Schematic illustration of charge carrier separation in a MoS2/g-C3N4/GO junction, and (c) Nyquist plots of MoS2 QDs, CNQDs, CNQDs/GO, and MoS2/4g-C3N4/GO. Conclusions In summary, we have demonstrated a simple technique to fabricate MoS2/g-C3N4/GO ternary nanostructures via a facile ion exchange process. The novel ternary composite is consisted of MoS2 QDs/CNQDs nanojunctions packed on the GO layer with exclusive contact of the g-C3N4 side. Benefiting from the formed type II heterostructure between MoS2 QDs and CNQDs, the efficient collection of electrons in MoS2 and holes in g-C3N4 is achieved, and therefore leading to the fast separation of photoinduced charge carriers at the interface. Furthermore, the holes in g-C3N4 can transport into GO speedily owing to the tight contact between CNQDs and GO, which enables a further effective suppression in the recombination of photogenerated electron-hole pairs. Hence, the photocurrent density and H2 evolution rate of MoS2/g-C3N4/GO ternary composite are significantly enhanced compared with MoS2/g-C3N4 photocatalyst. The current work provides a novel strategy to the design and synthesis of high performance composite photocatalysts for water splitting.

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Supporting Information

ICP-AES of MoS2/g-C3N4/GO composites, EDX of MoS2/4g-C3N4/GO composite, XPS and FTIR of GO, Zeta potential of GO and g-C3N4. The material is available free of charge via the Internet at http://pubs.acs.org Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant 61376066, 21401203), and CAS “Light of West China” Program. References (1) Fujishina, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37-41. (2) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial photosynthesis for solar water-splitting. Nat. Photonics 2012, 6, 511-518. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446-6473. (4) Maeda, K. Photocatalytic water splitting using semiconductor particles: History and recent developments. J. Photochem. Photobiol. C 2011, 12, 237-268. (5) Li, H.; Zhou, Y.; Tu, W.; Ye, J.; Zou, Z. State-of-the-art progress in diverse heterostructured photocatalysts toward promoting photocatalytic performance. Adv. Funct. Mater. 2015, 25, 998-1013. (6) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002, 297, 2243-2245. (7) Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.; Galli, G.; Wang, F. Emerging photoluminescence in monolayer MoS2. Nano Lett. 2010, 10, 1271-1275.

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Table of Contents

With a facile ion exchange process, we have rationally designed the MoS2/g-C3N4/GO ternary nanostructures for efficient H2 evolution (G = generation, S = separation, T = transport, R = recombination).

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GO Ternary Heterojunction with

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