Abstract

A 2D porous graphitic C3N4 nanosheets/nitrogen-doped graphene/layered MoS2 ternary nanojunction is synthesized using a simple pyrolysis process followed by a hydrothermal treatment. The 2D ternary nanojunction exhibits significantly enhanced photoelectrochemical and photocatalytic activities due to the large contact area, efficient light absorption, and rapid charge separation and transport. Two-dimensional (2D) nanosheets have attracted considerable attention recently owing to their unique electronic and optical properties,1 as well as their promising applications in solar cells, water splitting, and environmental purification.2 Among various 2D nanosheets, the graphitic carbon nitride (g-C3N4) nanosheet is especially attractive because of its appropriate bandgap, large surface area, and excellent chemical stability, making it potentially suitable for conversion of solar energy to electricity or chemical energy.3 However, the efficiency of g-C3N4 nanosheets is far from satisfaction thanks to the high recombination rate of photogenerated electron–hole pairs. Coupling g-C3N4 nanosheets with other layered semiconductors to form a nano/heterojunction is an effective strategy for improving the photoconversion efficiency.4 The construction of layered junction increases the contact area for efficient charge transfer across the interface compared with 0D nanoparticles that are only in point contact. In addition, the layered junction shortens the charge transport time and distance, thereby promoting the separation of electron–hole pairs and leading to enhancement of overall photoconversion efficiency.5 Among various layered semiconductors, few-layer MoS2 is probably one of the most ideal candidates owing to its suitable band-edge positions6 and good lattice matching with g-C3N4,7 which can enhance light harvesting and promote photogenerated charge separation across the interfaces. Although MoS2 has been widely investigated as an electrocatalyst for hydrogen evolution,7, 8 the utilization of the 2D layered MoS2 as a photocatalyst has not been adequately addressed.6 Similarly, 2D graphene shows many intriguing electronic and optoelectronic properties and has attracted much attention recently.9 In particular, it can be used as an electron mediator for shuttling electrons, leading to effective separation of photogenerated carriers at the junction interface.10 Feng et al.11 and Amal et al.12 recently demonstrated that reduced graphene oxide could improve the photoelectrochemical and photocatalytic water splitting performance of 0D/1D (nanoparticles/nanorods) and 0D/0D (nanoparticles/nanoparticles) hybrids, respectively, by providing low-resistance pathways for shuttling photogenerated electrons and holes at interfaces. Moreover, it is worth noting that unique properties of the graphene/MoS2 interface, where electrons can rapidly transport between graphene and MoS2, have been highlighted.13 However, to the best of our knowledge, there is no study on a 2D layered nano/heterojunction with graphene or its derivatives as electron mediators for the purpose of solar conversion. Herein, we present the fabrication and characterization of 2D porous g-C3N4 nanosheets/nitrogen-doped graphene/layered MoS2 (CNNS/NRGO/MoS2) ternary nanojunction. Such an architecture provides a broadening optical window for effective light harvesting, short diffusion distance for excellent charge transport, as well as a large contact area for fast interfacial charge separation and photoelectrochemical reactions. As a result, the nanojunction is anticipated to exhibit good photoelectrochemical and photocatalytic activities under visible light. The overall synthetic procedure of the CNNS/NRGO/MoS2 nanojunction is illustrated in Figure 1a and Figure S1, Supporting Information. Nitrogen-doped partially reduced graphene oxide (NPRGO)-based CNNS were first fabricated through pyrolysis of urea on the surface of GO at 550 °C under an argon atmosphere. During this process, the nitrogen-containing species14 were released from the polycondensation of urea, which led to the partial reduction of GO and nitrogen doping in GO. Subsequently, the CNNS/NPRGO hybrid was dispersed in an aqueous solution of sodium molybdate and thiourea, and placed in an autoclave for hydrothermal treatment. In this way, NPRGO was gradually reduced to NRGO simultaneously with the dispersion of MoS2 sheets onto the surface of CNNS/NRGO nanosheets (see Supporting Information for details). A field emission scanning electron microscopy (FESEM) study shows that the bare CNNS is porous and consists of many curved nanosheets with lamellar morphology (Figure 1b,c). The average thickness of the nanosheets is around 14 nm (Figure 1d), corresponding with about a few dozen stacked layers. Note that some hollow spherical structures are observed owing to partial interconnection and overlapping of the curved nanosheets (Figure 1d). For CNNS/NPRGO, the surfaces of CNNS are uniformly covered by NPRGO sheets that have a crumpled-shell structure (Figure 1e–f). Coupled with the atomic force microscopy (AFM) analysis (Figure S2, Supporting Information), it is clear that the number of layers in NPRGO nanosheets should be less than 8. Additionally, an intimate interfacial contact between CNNS and NPRGO sheets is clearly observed (Figure 1e, inset), which is beneficial for fast charge transfer across the interface. After the hydrothermal treatment, MoS2 sheets have been successfully introduced into the hybrid without obvious aggregation (Figure 1g). The corresponding elemental mapping analysis clearly shows the well-defined spatial distribution of all elements C, N, S, Mo, and O (Figure S3, Supporting Information), confirming the formation of the CNNS/NRGO/MoS2 nanojunction with an intimate contact between them. Thermogravimetric analysis (TGA) shows that the amount of NRGO present in the nanojunction is about 6.61 wt% (Figure S4, Supporting Information). Further information about the microstructure of the CNNS/NRGO/MoS2 nanojunction was obtained from transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images. TEM images of CNNS show typical layered platelet-like morphology with obvious porous structures (Figure 2a,b), which are created by ammonia-based gas bubbles during the pyrolysis process of urea. The released gas favors the expansion of stacked layers, leading to the formation of few-layer nanosheets and porous structures.15 The selected area electron diffraction (SAED) pattern and the HRTEM image indicate that the nanosheets are amorphous (Figure 2c,d). Figure 2e,f show TEM images of the CNNS/NPRGO with multiple overlapping layers, demonstrating that the hybrid well maintains the 2D sheet structure. Since CNNS is sandwiched between NPRGO nanosheets through polymerization of urea pre-adsorbed on the GO sheets, the hybrid possesses a more compact sheet-on-sheet structure. After the MoS2 coating, the parallel-aligned dark lines (blue arrows) corresponding with different layers of MoS2 sheets are observed (Figure 2g–h). The MoS2 sheets grow along the surface of CNNS/NRGO and layered contacts among CNNS, RGO, and MoS2 are clearly seen. The SAED pattern (Figure 2i, inset) of the CNNS/NRGO/MoS2 nanojunction shows several diffraction rings indexed to the planes of hexagonal-phase MoS2 and NRGO, respectively.6, 16 An HRTEM image shows that the NRGO sheets consist of ≈5 layers, and the numbers of MoS2 layers is approximately 8–15 with lattice spacing of 0.62 nm (Figure 2i, inset). Importantly, the HRTEM image also shows a distinguished and coherent interface among the CNNS, NRGO, and MoS2, indicating that a nanojunction might be formed, which could result in better separation of photoinduced charge carriers and more efficient electron transfer within the composite structure. The crystal structure of the obtained samples was investigated by X-ray diffraction (XRD). Compared with bulk g-C3N4 (BCN), the (002) peak of CNNS at 27.29° (inter-planar stacking peak of conjugated aromatic systems) displays a markedly reduced intensity and dramatically broadened width (Figure 3a). Together with near absence of the (100) peak at 13.55° (in-plane structural packing motif),17 it suggests that the CNNS possesses layered structures.3 With the introduction of NPRGO, the broad peak becomes stronger, which may be due to the overlapping from two diffraction peaks of CNNS (002) and NPRGO (002). However, the presence of NPRGO can be discerned by Raman and infrared spectroscopy, as discussed later. For the nanojunction, all of the diffraction peaks can be readily indexed to the CNNS and MoS2 (JCPDS 37–1492). MoS2 exhibits a hexagonal crystal structure, and no impurity is observed. According to the Scherrer formula,18 the dimension of the MoS2 in the nanojunction along the z-axis is 5.39 nm based on the half peak width of the (002) peak, corresponding with approximately eight layers of MoS2. This result is in good agreement with the observation from HRTEM images. Further insights into structural properties of the CNNS/NRGO/MoS2 nanojunction were obtained from Raman spectroscopy (Figure 3b). The typical Raman spectrum of the nanojunction exhibits two remarkable peaks at 1340 and 1592 cm−1, corresponding with the well-documented D (k-point phonons of A1g symmetry) and G bands (E2g phonon of Csp2 atoms), respectively.19 The hydrothermal treatment resulted in a down-shift of G bands from 1608 to 1592 cm−1 compared with that of CNNS/NPRGO, indicating more graphene domains formed on NRGO sheets, which enhances the conductivity of NRGO sheets and thus facilitates the charge transfer.20 Likewise, the down-shift of the G band (from 1613 to 1608) was also observed for the CNNS/NPRGO as compared with GO, which may result from the structural defects and edge plane exposure caused by nitrogen doping21 and partial removal of oxy-functional groups on GO (see below). For nitrogen doping, previous studies showed that the substitution of carbon atoms with "graphitic nitrogen" caused n-type doping, which can result in the down-shift of the G band.22 Therefore, the downshift of G band in our observation probably indicates a relatively high content of graphitic nitrogen doping. Infrared spectroscopy (Figure S5, Supporting Information) reveals that almost all of the oxygen-containing functional groups (C=O stretching vibrations of epoxy groups at 1075 cm−1 and C–O stretching vibrations of COOH groups at 1724 cm−1) of CNNS/NPRGO disappear after the hydrothermal procedure, indicating that the reduction of NPRGO to NRGO is complete.23 Notably, an enhanced absorption band at 1625 cm−1 should be attributed to the overlapping between the stretching vibration of C–N and the stretching vibration of the C–C of NRGO. Besides, in the nanojunction, the breathing mode of triazine units at 809 cm−1 exhibits a slight shift towards high wavenumber compared with that of CNNS/NPRGO, suggesting that the hybridization may have occurred between CNNS/NPRGO and MoS2 sheets.24 To clarify this issue further, X-ray photoelectron spectroscopy (XPS) analysis was performed. The survey XPS spectrum of the nanojunction reveals the presence of C, N, Mo, S, and O elements with an Mo/S atomic ratio of ≈1:2, which is in good agreement with the nominal atomic composition of MoS2 (Figure S6, Supporting Information). The high-resolution C 1s spectrum of GO displays five fitted peaks at 284.3, 285.2, 286.7, 288.1, and 289.5 eV, which are assigned to the C=C, C=OH, C=O=C, C–O, and O=C–O configurations, respectively (Figure S7a, Supporting Information).25 The constituents of oxygen functionalities decrease after being treated with urea and a new peak at 287.9 eV attributed to the C=N bond is observed for the CNNS/NPRGO (Figure S7b, Supporting Information), suggesting moderate reduction of GO to RGO and the presence of carbon nitride in the hybrid.3, 26 Obviously, the amount of oxygen-functional groups diminished greatly after further hydrothermal treatment (Figure 3c), indicating that most oxygenate groups were efficiently removed, which is beneficial to the charge separation between CNNS and MoS2 sheets. The corresponding high-resolution N 1s spectrum of the nanojunction can be deconvoluted into five peaks centered at 396.5, 398.1, 399.2, 400.7, and 404.8 eV, corresponding to N-graphene, pyridine N, pyrrolic N, graphitic N, and N-"O" (oxidized nitrogen), respectively (Figure 3d).27 The N-graphene peak may be attributed to the nitrogen bonding between CNNS and NRGO, which can serve as a bridge linking CNNS with NRGO.28 Compared with CNNS/NPRGO (Figure S7c,d, Supporting Information), the percentage of graphitic N peak increased from 10.1 to 14.5% for the nanojunction, indicating that more graphitic N was formed in the nanojunction. This finding is attributed to the fact that the decomposition of urea first induced the incorporation of N into the edges and defect sites of graphene forming pyridine N and pyrrolic N.29 During the subsequent hydrothermal reaction, these N atoms could be transformed to the graphitic N, improving electrical conductivity of NRGO.21 This result is consistent with a previous report.29 The optical properties of the samples were studied by UV-vis absorption spectroscopy (Figure 3e). The CNNS exhibits a slight blue-shift of the absorption edge (441 nm) together with an increased absorption intensity compared with BCN (absorption edge 450 nm) owing to the quantum confinement30 and light scattering effects31 of the nanosheet structure (Supporting information, Figure S8). The NPRGO loading enhances its light absorption over the entire range of wavelengths investigated; this is a typical behavior of graphene.32 Significant enhancement in light absorption intensity and an obvious red-shift of the absorption edge are observed for the CNNS/NRGO/MoS2 nanojunction, indicating that the loading of MoS2 further improves the light absorption ability of the composite owing to its narrower band gap.33 The results suggest that the nanojunction may be able to absorb more light to produce electron–hole pairs and thus exhibit improved catalytic activity. Drastic quenching of photoluminescence intensity of CNNS was observed after the addition of NRGO and MoS2 (Figure 3f), implying that the photogenerated electrons and holes have better separation in the nanojunction. This phenomenon is attributed to the efficient charge transfer between CNNS and MoS2 through the NRGO interlayer (detailed analysis is shown below), which prevents the direct recombination of electrons and holes.[13] The transient photocurrents of the samples were measured during repeated ON/OFF illumination cycles at 0.8 V. All of the samples exhibit prompt and reproducible photocurrent responses on each illumination (Figure 4a). When the irradiation was interrupted, the photocurrent rapidly dropped to almost zero (steady-state value), and the photocurrent reverted to the original value once light was switched back on again. The transient photocurrent density of the CNNS/NPRGO is more than 19.02 μA cm−2, whereas that of the CNNS is less than 9.06 μA cm−2. The photocurrent density of the CNNS/NPRGO is about 2.09 times higher than that of the CNNS. This result confirms the constructive effect of the NPRGO in promoting electron shuttling and suppressing charge recombination, where NPRGO acted as an electron transfer channel, transferring the photogenerated electrons from CNNS. Although bulk MoS2 has poor photocatalytic/photoelectrochemical activity,34 the as-prepared pure MoS2 sheet exhibits a transient photocurrent density of 2.54 μA cm−2, which can be attributed to the quantum confinement effect.35 This effect can widen the electronic band gap of MoS2, improving the redox activity of photogenerated electrons and holes. Note that the CNNS/NRGO/MoS2 nanojunction exhibits the highest transient photocurrent density among the five samples tested. The photocurrent density reaches 27.76 μA cm−2, which is 8.14-, 3.06-, 1.46-, and 10.93-fold larger than those of BCN (3.41 μA cm−2), CNNS, CNNS/NPRGO, and MoS2, respectively. This enhancement of photocurrent density probably resulted from the enhanced light harvesting owing to the loading of MoS2 sheets and more efficient separation of photogenerated electron–hole pairs owing to the formation of NRGO-based ternary layered heterojunctions. To understand further the role of CNNS, NRGO interlayers, and MoS2 sheets in the photoelectrochemical performance, typical current–voltage measurements of various samples were performed (Figure 4b). The CNNS shows a higher photocurrent density of 10.98 μA cm−2 at 0.9 V, which is about 2.57 times higher than that of BCN (4.27 μA cm−2). This can be ascribed to the fact that the unique geometry of the nanosheet increases the contact area and shortens the distance that photogenerated charges diffuse to reaction sites, effectively suppressing the recombination of photogenerated electron–hole pairs.5 Both the CNNS/NRGO/MoS2 nanojunction (37.63 μA cm−2) and the CNNS/NPRGO nanojunction (23.23 μA cm−2) show a significant enhancement in photocurrent density after the NRGO/NPRGO loading. These current densities are about 1.46 and 2.12 times higher than those of CNNS/MoS2 (25.69 μA cm−2) and CNNS (10.98 μA cm−2) at 0.9 V, respectively. Assuming that the number of electron–hole pairs generated in the CNNS/NRGO/MoS2 nanojunction with and without NRGO is comparable during the light irradiation, the difference in photocurrent density could be explained by the reduction of electron–hole recombination achieved in the CNNS/NRGO/MoS2 nanojunction owing to the presence of NRGO, indicating that the NRGO interlayers in the nanojunction have an important role in promoting separation and transfer of photogenerated carriers. In addition, the unique multilayer structure can further shorten the photoelectron transport distance, thus facilitating fast electron transfer across the interface.7 Moreover, the photocurrent value of the CNNS/NRGO/MoS2 nanojunction is much higher than that of physical mixture of CNNS/NRGO and MoS2 (25.24 μA cm−2), indicating that the intimate contact between CNNS/NRGO and MoS2 is crucial for the interface charge transfer. Combined with the HRTEM analysis (Figure 2i), it can confirm the formation of a nanojunction. When the ternary nanojunction is formed, the space charge layer creates a built-in electric field and separates the electrons and holes through the NRGO interlayer on light illumination, resulting in an enhancement of photocurrent density.36 These results suggest that the enhanced light harvesting (Figure 3e), efficient separation of photogenerated electron–hole pairs, and the layered nanostructure of the CNNS/NRGO/MoS2 nanojunction have a significant role in improving the overall photoelectrochemical performance. Although the obtained photocurrent density of the CNNS/NRGO/MoS2 nanojunction is hard to compare directly with reported values because of different test conditions, it is relatively high among all reported g-C3N4-based composite nanostructures with photocurrent densities ranging from 0.75 to 150 mA cm−2 (note that the large photocurrent density of 150 mA cm−2 was obtained using the K4Fe(CN)6 solution as an electrolyte, which increased the charge separation efficiency chemically by reacting Fe3+ with electrons).37 Considering that preparation parameters of the nanojunction have not been fully optimized, these results are very encouraging. To gain deeper insights into the charge transport behavior in the nanojunction system, we conducted electrochemical impedance spectroscopy (EIS) measurements (Figure 4c). In the Nyquist diagram, the radius of each arc is associated with the charge-transfer process at the corresponding electrode/electrolyte interface with a smaller radius corresponding with a lower charge-transfer resistance.38 The CNNS/NRGO/MoS2 nanojunction exhibits the smallest charge transfer resistance among all CNNS-based electrodes both in dark and under irradiation, suggesting that effective shuttling of charges between the electrode and the electrolyte, and faster interfacial charge transfer occurred at the composite interface owing to the formation of the nanojunction.11 Moreover, no significant photocurrent change was observed within 1400 s of illumination, indicating that the nanojunction is stable (Figure 4d). The photocatalytic activities of the samples were evaluated by simultaneous oxidation of methylene blue (MB) and reduction of Cr(vi). Although the CNNS/NPRGO possesses a higher specific surface area (66 m2 g−1, supporting information, Figure S9, Supporting Information), the kinetic constants for the oxidation of MB and the reduction of Cr(vi) are, respectively, enhanced by 141 and 265% when the photocatalyst of CNNS/NPRGO was replaced with the CNNS/NRGO/MoS2 nanojunction (Figure 4e–f), indicating that the enhanced photocatalytic activity is attributed to the efficient light absorption and the formation of a nanojunction. The formation of the NRGO-based layered nanojunction benefits the separation of photoinduced charge carriers through the internal electrostatic field in the junction region, so that more holes and electrons could participate in reactions instead of recombination. On the basis of the above results, a tentative charge transfer mechanism for the CNNS/NRGO/MoS2 nanojunction is proposed in Scheme S1, Supporting Information. On irradiation, electrons are promoted from the valence bands (VBs) of CNNS and MoS2 to their respective conduction bands (CBs). Owing to the band alignment and the potential difference,6, 32, 39 the photogenerated electrons on the CB of CNNS can be transferred easily through the NRGO sheets (which act as a conductive electron transport "freeway")12 to the CB of MoS2 sheets, where photoreduction reactions occur.7 Simultaneously, NRGO interlayers also provide pathways for photogenerated holes at the VB of MoS2 to reach the VB of CNNS,[13] and finally the holes are consumed by photooxidation reactions, promoting the effective charge separation. Of course, direct transfer of some photogenerated electrons and holes between CNNS and MoS2 is possible.32 Moreover, the nanojunction formed among CNNS, NRGO, and MoS2 can effectively retard the photogenerated electron–hole recombination and prolong the lifetime of electron–hole pairs owing to the internal electric field formed by the space charge layer, leading to the enhanced photoelectrochemical and photocatalytic activity. In summary, we have rationally designed a 2D ternary nanojunction consisting of porous CNNS nanosheets, NRGO interlayers, and layered MoS2 for efficient solar conversion. In this unique 2D ternary nanostructure, the CNNS with a large surface area absorbs visible light, and layered MoS2 enhances light absorption to generate more photoelectrons and simultaneously promotes charge separation and transfer at CNNS/MoS2 interfaces (sheet to sheet), while NRGO interlayers work as the electron mediator for shuttling electrons–holes between CNNS and MoS2 sheets. As a result, it exhibits a higher photocurrent density and photocatalytic activity for simultaneous oxidation of MB and reduction of Cr(vi) than other reference electrodes under simulated sunlight irradiation. Our findings pave the way to build reliable 2D layered triple-nanojunction materials for photoelectrochemical applications. Financial support for this work was provided by the US National Science Foundation (ECCS-1001039), the US Department of Energy (DE-EE0003208), and the Research Growth Initiative Program of the University of Wisconsin-Milwaukee (UWM). As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. 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