GO Ternary Heterojunction with

Aug 15, 2017 - University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, P. R. China. § Qingdao Center of Resource Chemistry &...
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Research Article pubs.acs.org/journal/ascecg

Facile Synthesis of MoS2/g‑C3N4/GO Ternary Heterojunction with Enhanced Photocatalytic Activity for Water Splitting Min Wang,†,‡ Peng Ju,† Jiajia Li,†,‡ Yun Zhao,†,‡ Xiuxun Han,*,†,§ and Zhaomin Hao∥ †

Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, P. R. China ‡ University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, P. R. China § Qingdao Center of Resource Chemistry & New Materials, 36 Jinshui Road, Qingdao 266100, P. R. China ∥ Henan University, 85 minglun Road, Kaifeng 410200, P. R. China S Supporting Information *

ABSTRACT: On the basis of a simple ion exchange method, a MoS2/gC3N4/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 water-splitting redox activities. The designed heterostructure featured a 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. KEYWORDS: MoS2, C3N4, GO, Heterojunction, Water splitting

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separation.14−19 Recently emerged studies on ternary junctions such as Ag2CO3/Ag/AgBr,20 CdS/Au/ZnO,21 and CNNSs (gC3N4 nanosheets)/NRGO (nitrogen-doped graphene)/MoS222 have gained increasing research attention. In the 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 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 it 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,

INTRODUCTION Since Fujishima and Honda reported hydrogen production by the 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 researchers have paid attention to perfecting 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, promoting the separation of charge carriers is urgently needed to further enhance the photocatalytic performance in photocatalysis. Constructing a heterostructure between semiconductors is one of the 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 © 2017 American Chemical Society

Received: May 3, 2017 Revised: July 31, 2017 Published: August 15, 2017 7878

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

Research Article

ACS Sustainable Chemistry & Engineering

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; and the corresponding elemental mapping images of C (g), N (h), Mo (i), and S (j) in MoS2/4g-C3N4/GO. Inset in panel b: The size distribution of the as-prepared CNQDs evaluated from the TEM image by measuring about 100 individual dots. Ground 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 a yellowish solution of CNQDs was obtained. MoS2/g-C3N4/GO composite was synthesized by two steps. First, CNQDs/GO was prepared by stirring 2 mL of CNQDs (0.14 g L−1) with 2.0 mL of GO (5.0 g L−1) for 3 h at room temperature. The obtained samples were labeled as 2CNQDs/GO, where 2 refers to the volume in milliliters of CNQDs. Subsequently, 3CNQDs/GO, 4CNQDs/GO, and 5CNQDs/GO samples were obtained via the same process with 3, 4, and 5 mL of 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 of 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 nine times at 15 000 rpm, 0.1 g of TAA (CH3CSNH2) was added and the mixture 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/gC3N4 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 JEM2100 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 Bruker D8 Advanced powder diffractometer using Cu Kα radiation (λ = 0.154 06 nm). UV−vis absorption spectra of the samples were

g-C3N4 quantum dots (CNQDs), and MoS2 QDs. By reasonably optimizing the location of MoS2, C3N4, and GO, both GO and MoS2 can only contact 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 CNNSs, which tend to fold, CNQDs are proposed for use with the following conjunct effects: (1) the improved interfacial contact between g-C3N4 and GO and (2) the facilitated transfer of photogenerated holes from g-C3N4 to GO acceptor. The asprepared MoS2/g-C3N4/GO ternary heterojunctions are investigated in terms of morphology, composition, crystal structure, photocurrent, and photocatalytic response, and they exhibit significantly enhanced photoelectrocatalytic and photocatalytic activities.

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EXPERIMENTAL SECTION

Synthesis of Photocatalysts. The graphene oxide (GO) was synthesized through chemical exfoliation of graphite powders according to the Hummers’ method.24 For a typical synthesis of GO, 1.0 g of graphite powder was dispersed into 23 mL of concentrated H2SO4 under an ice bath. A 3.0 g portion of KMnO4 was added gradually under vigorous stirring. The mixture was ultrasonicated at room temperature for 10 h until thoroughly dispersed. Then 46 mL of H2O and 150 mL of 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 rpm 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 a powder for further processing and characterization.9 7879

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

Research Article

ACS Sustainable Chemistry & Engineering

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) N 1s, (e) Mo 3d, and (f) S 2p for MoS2/4g-C3N4/GO. recorded on a Shimadzu UV-3600 apparatus. The structural information for the samples was measured by a Fourier transform spectrophotometer (FT-IR, Bruker Tensor 27) with KBr as the reference sample. The ζ-potentials of 0.5 mg mL−1 particle suspensions were obtained from a Zeta PALS instrument (Brookhaven Instruments Co.). The 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 microscopy (AFM) images were acquired using a Digital Instrument Shimadzu SPM-9600 (The sample was prepared by drop-casting a 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 equipped with a thermal conductive detector and high-purity Ar as the carrier gas. For the stability test, MoS2/4g-C3N4/GO catalyst was withdrawn at regular intervals (5 h).

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). A typical TEM image of MoS2/4g-C3N4/GO (Figure 1d) shows that rich CNQDs can be seen on the gauzelike substrate, and 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 of the Supporting Information (SI), 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 MoS2/4gC3N4/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 in the MoS2/g-C3N4/GO composites is further confirmed 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, SI) are well consistent with the construction of the MoS2/g-C3N4/GO heterojunction observed from the TEM, HRTEM image, and the SAED pattern. XRD Analysis. 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-

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RESULTS AND DISCUSSION TEM Analysis. 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 the 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 7880

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

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ACS Sustainable Chemistry & Engineering

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, 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, SI). Besides, only C element is detected by EDX (Figure S6b, SI) and XPS (Figure S6c, SI), elucidating that MoS2 cannot directly deposit on GO films due to the numerous 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. 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 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 inherited from the GO is also evident.29 The photoelectrochemical (PEC) performances of the asprepared 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 the hydrogen evolution 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/gC3N4, 183 mV for CNQDs, and 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 their appropriate band gap (1.82 eV). Comparably, MoS2/g-C3N4 electrode presents an increasing photocurrent due to the enhancement in the light absorption ability (Figure 3a), and the appreciably improved photocurrent is also owed to the enhancement of spatial charge separation due to the formation of a type II heterojunction structure at the interface between g-C 3N4 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 transport layer are beneficial for the 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

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 on the structure of MoS2/g-C3N4/GO composites, we conducted XPS analyses to further study the valence state of this photocatalyst. 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 S-related peaks, 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. On the basis of the above results, the formation mechanism of the MoS2/g-C3N4/GO composite is proposed, as illustrated in Scheme 1. There are abundant Scheme 1. Schematic Illustration of the Proposed Formation Mechanism for the MoS2/g-C3N4/GO Composite

oxygenated functional groups on the surface of GO derived from the oxidation process (Figure S3, SI), leading to a negative ζ-potential on the surface of GO (Figure S4, SI). Besides, with the assistance of the protonation due to acidic cutting, CNQDs exhibit positive potentials (Figure S5, SI). Resulting from the pretreatment, 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 a 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, 7881

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

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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.

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) CNQDs, physical-mixture sample of MoS2 QDs + CNQDs + GO and the mixture with hydrothermal treatment, MoS2/4g-C3N4/GO, and MoS2 QDs; and (d) MoS2/g-C3N4, MoS2 QDs, and MoS2/4g-C3N4/GO heterojunction.

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 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 spectral region than the individual

RHE) was chosen as the 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 the minority carrier.30 Besides, it can be seen that the transient photocurrent density 7882

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

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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 the photocatalytic reaction.

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, the 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 a 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 gC3N4 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 is 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 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 the 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 the 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 by XPS analyses. The reduction of GO was verified by the absence of C−OH, CO, and OC−OH configurations in the highresolution C 1s XPS spectra 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-C 3 N 4 and MoS 2 . First, g-C 3 N 4 demonstrates high stability due to its inherent structure.9

components to generate more electron−hole pairs, which is 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/gC3N4/GO composites show high stability because their photoinduced holes (h+) can migrate to the 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 composite can avoid the photocorrosion of sulfur during the solar watersplitting reaction. The elemental and XPS analyses on the fresh and used MoS2/4g-C3N4/GO were performed and compared to illustrate the stability of the composite. It can be seen in Table S1 (SI) 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 (SI) 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. 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 the corresponding host substrates because of its effective separation of photogenerated charge carriers. It can be seen in Figure 4b that the MoS2/gC3N4 content in the MoS2/g-C3N4/GO composite played a key role in achieving the high photocatalytic activity of the MoS2/gC3N4/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, SI). 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, SI). To demonstrate the necessity of tight interconnection for heterojunctions, the physically mixed sample of MoS2 QDs + CNQDs + GO and the mixture with hydrothermal processing 7883

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

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ACS Sustainable Chemistry & Engineering

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.

materials results 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 to then transfer to GO to form the charge-separated state (MoS2−/gC3N4/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/gC3N4/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.

Second, in this work MoS2 displays excellent stability resulting from the efficient transfer of its photoinduced holes (h+) to the GO side through the constructed heterojunction structure, thus avoiding photocorrosion of sulfur during solar water-splitting reaction. Third, 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 the catalytic H2 evolution reaction. Mechanism of the Enhanced Photocatalytic Performance in the MoS2/g-C3N4/GO System. On the basis of 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/gC3N4 interface, which plays a crucial role in the photocatalytic process. In addition, GO leads to 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, SI), respectively.31−33 Thus, the band edges of the conduction band (CB) of MoS2 QDs were 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, SI), which give values of flat band potential of −1.21 and −0.10 V for CNQDs and MoS2 QDs, respectively.9,10,38−40 Together with the band 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 the empirical equation. Meanwhile, the work function values of GO thin film samples were found to be 0.5 V (vs RHE),41 more negative than the VB of CNQDs. Therefore, once MoS2 and g-C3N4 are integrated together, the band alignment between the two

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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 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, 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 of the 7884

DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886

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ACS Sustainable Chemistry & Engineering

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recombination of photogenerated electron−hole pairs. Hence, the photocurrent density and H2 evolution rate of MoS2/gC3N4/GO ternary composite are significantly enhanced compared with MoS2/g-C3N4 photocatalyst. The current work provides a novel strategy for the design and synthesis of high-performance composite photocatalysts for water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01386. AFM images and corresponding height curves of CNQDs and MoS2 QDs; ICP-AES of MoS2/g-C3N4/ GO composites; EDX of MoS2/4g-C3N4/GO composite; XPS and FTIR of GO; ζ-potential of GO and g-C3N4; TEM, EDX, and XPS of MoS2/GO synthesized by a parallel process; S 2p XPS of fresh and used MoS2/4gC3N4/GO; elemental analysis of MoS2/g-C3N4/GO samples; TEM image of MoS2/5g-C3N4/GO; and Tauc and Mott−Schottky plots of MoS2 QDs and CNQDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-931-4968054. Fax: +86-931-4968054. E-mail: [email protected]. ORCID

Xiuxun Han: 0000-0002-1696-0925 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 61376066, 21401203) and CAS “Light of West China” Program.

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DOI: 10.1021/acssuschemeng.7b01386 ACS Sustainable Chem. Eng. 2017, 5, 7878−7886