NiS Multiple

Research Article pubs.acs.org/journal/ascecg

Fabricating the Robust g‑C3N4 Nanosheets/Carbons/NiS Multiple Heterojunctions for Enhanced Photocatalytic H2 Generation: An Insight into the Trifunctional Roles of Nanocarbons Jiuqing Wen,†,‡,§ Jun Xie,†,‡,§ Zhuohong Yang,†,§ Rongchen Shen,†,‡,§ Huiyi Li,†,‡,§ XingYi Luo,†,§ Xiaobo Chen,∥ and Xin Li*,†,‡,§ †

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, P. R. China § Key Laboratory of Energy Plants Resource and Utilization, Ministry of Agriculture, Key Laboratory of Biomass Energy of Guangdong Regular Higher Education Institutions, Institute of New Energy and New Materials, South China Agricultural University, Guangzhou 510642, P. R. China ∥ Department of Chemistry, University of Missouri − Kansas City, Kansas City, Missouri 64110, United States

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‡

ABSTRACT: In this work, robust nanocarbons, including graphite (G), carbon nanotube (CNT), reduced graphene oxide (rGO), carbon black (CB), and acetylene black (AB), have been successfully coupled into the interfaces between g-C3N4 and NiS using a facile precipitation method. The results demonstrated that nanocarbons played trifunctional roles in boosting the photocatalytic H2 evolution over g-C3N4, which can not only act as effective H2-evolution co-catalysts but can also serve as conductive electron bridges to collect photogenerated electrons and boost the H2-evolution kinetics over the NiS co-catalysts. More interestingly, the nanocarbons can also result in the downshift of valence band of g-C3N4, thus facilitating the fast oxidation of triethanolamine and charge-carrier separation. Particularly, in all five ternary multiheterostructured systems, the g-C3N4-0.5%CB-1.0% NiS (weight ratio) and g-C3N4-0.5%AB-1.0%NiS photocatalysts exhibited the highest H2-evolution rates of 366.4 and 297.7 μmol g−1 h−1, which are 3.17 and 2.57 times higher than that of g-C3N4-1.0%NiS, respectively. Apparently, the significantly enhanced H2-evolution activity of multiheterostructured g-C3N4/carbon/NiS composite photocatalysts can be mainly ascribed to the trifunctional nanocarbons, which serve as the conductive electron bridges rather than the general co-catalysts. More importantly, it is revealed that the amorphous carbons with higher electrical conductivity and weaker electrocatalytic H2-evolution activity are more suitable interfacial bridges between g-C3N4 and NiS co-catalysts for maximizing the H2 generation. This work may give a new mechanistic insight into the development of multiheterostructured g-C3N4-based composite photocatalysts using the combination of trifunctional nanocarbon bridges and earth-abundant co-catalysts/semiconductors for various photocatalytic applications. KEYWORDS: Photocatalytic hydrogen evolution, NiS co-catalysts, g-C3N4, Trifunctional nanocarbons, Graphene

1. INTRODUCTION As is well known, heterogeneous semiconductor photocatalytic hydrogen production driven by renewable solar energy could simultaneously achieve the directly sustained solar-to-fuel conversion and solve environmental pollution, thus gaining a significant amount of research interest around the world.1,2 Since Fujishima and Honda first discovered the photoelectrochemical overall water splitting over a n-TiO2 cell,3 numerous efforts have been devoted to exploit a wide variety of semiconductors for photocatalytic H2 generation from water splitting in the past decades.2,4−10 Among various types of heterogeneous semiconductors, the metal-free polymeric g-C3N4, with a medium band gap of 2.7 eV has been identified as one of the exciting visible-light-driven photocatalysts for solar fuel production11−16 because it possesses the favorable conduction band bottom and the proper valence band top (−1.3 and 1.4 V vs NHE at pH 7) © 2017 American Chemical Society

for reduction of H2O and CO2 and oxygen evolution, respectively. Clearly, it has promptly become a shining star in photocatalytic hydrogen production since the first report in 2009,17 owing to its unique chemical and thermal stability, easy fabrication from the condensation of simple nitrogen-rich precursors, low toxicity, and low cost.15,18 Nevertheless, the photocatalytic H2-evolution efficiency of g-C3N4 is mainly constrained by several typical challenges, including the rapid carrier recombination rate, moderate water oxidation ability, sluggish kinetics, insufficient active sites, and visible-light absorption, which greatly restrict its practical applications in photocatalytic H2 generation.19 Thus, it is highly desirable to develop some new Received: October 14, 2016 Revised: December 15, 2016 Published: January 16, 2017 2224

DOI: 10.1021/acssuschemeng.6b02490 ACS Sustainable Chem. Eng. 2017, 5, 2224−2236

Research Article

ACS Sustainable Chemistry & Engineering

also could reveal the rate-determining steps, which is favorable for the design and optimization of highly efficient g-C3N4-based composite photocatalysts. Therefore, in this study, the robust conductive nanocarbons, including graphite (G), carbon nanotube (CNT), rGO, carbon black (CB), and acetylene black (AB), will serve as efficient co-catalysts and electron transfer bridges for enhancing the photocatalytic H2-evolution activity over the g-C3N4/NiS hybrids. The ternary hybrids are synthesized by a simple and facile wet-chemistry method: ultrasonic dispersion treatments and the subsequent precipitation process (as shown in Scheme 1).

modification strategies to address these problems and improve the photocatalytic H2-evolution efficiency of g-C3N4. To this end, efficient engineering of the structure, composition, texture, surface, and interface of g-C3N4 has been demonstrated to be crucial to design high-performance g-C3N4-based photocatalysts for hydrogen generation under visible-light irradiation.2,15,20−26 It is well known that the charge separation and transfer and surface charge utilization play major roles in determining efficient photocatalysis.2,27,28 However, simultaneous optimization of the charge carrier separation, migration, and utilization of g-C3N4 is still very challenging. To date, it is well known that loading suitable co-catalysts on g-C3N4 has extensively proven to be one of the most promising strategies for accelerating its sluggish surface H2-elvolution rate. Considering the sustainable development and related economic issues, it is well accepted that the search for earth-abundant H2-evolution electrocatalysts, such as nonprecious metal-based, metal-free, and hybrid materials, have been found to be extremely important for achieving robust electrocatalytic hydrogen generation.29−34 More importantly, these electrocatalysts could be selectively deposited onto various nanostructured heterogeneous semiconductors as water-reduction co-catalysts to significantly boost the H2-evolution activity in aqueous solutions containing proper electron donors.2,35,36 Thus, far, various noble metal-free (such as N-doped graphitic carbon,37 CoS,38 Cu(OH)2,39 NiSx,27,40−45 Ni(dmgH)2,46 Ni(OH)2,47,48 NiO,49 MoS2,50,51 WS2,52,53 and FeOx54) H2-evolution electrocatalysts have been employed as co-catalysts to enhance the photocatalytic H2 generation over g-C3N4.55 However, for these earth-abundant co-catalysts, there have been also two obvious disadvantages: one is their weak electrical conductivity and low work function, as compared to the famous noble metal Pt, thus unfavorable for their effective collection and utilization of photogenerated electrons; the other is their well-known p-type semiconductor characteristics with much smaller band gaps, leading to the formation of typical p−n heterojunctions after the band-structure alignment under irradiation, which are specifically unfavorable for the transfer of photoexcited electrons from the conduction band of n-type g-C3N4 to the p-type co-catalysts. Therefore, to further enhance the activity and stability of the g-C3N4/noble metal-free co-catalysts hybrids, it is of great significance to improve the interfacial coupling to maximize the charge separation and transfer of hot electrons and the H2-evolution efficiency over p-type co-catalysts. More interestingly, despite the significant advances in the promoted photocatalytic H2 evolution over CdS and TiO2 semiconductors through combing the rGO, other various kinds of earthabundant co-catalysts such as MoS2,56,57 WS2,58 Ni(OH)2,59 and NiS,60 limited studies have been reported on the photocatalytic H2 evolution over the ternary hybrids of g-C3N4/ conductive carbons/earth-abundant co-catalysts.40,44 Thus, it would be of great interest to enhance the photocatalytic H2 evolution over the ternary g-C3N4/carbon/earth-abundant co-catalysts by using various nanocarbons with different structures and conductivity properties. To the best of our knowledge, the effects of nanocarbons with different surface physicochemical properties on the photocatalytic H2 evolution over the g-C3N4/NiS hybrids have been rarely reported, especially for some conductive amorphous carbons such as carbon black (CB) and acetylene black (AB). Importantly, more detailed studies on the mechanisms of charge-separation and photocatalytic activity enhancement over the ternary multiheterostructured hybrids with different carbon bridges

Scheme 1. Schematic Illustration for Fabrication of Ternary g-C3N4/Carbon/NiS Composites

The trifunctional roles of nanocarbons in boosting the photocatalytic H2 evolution over g-C3N4 are carefully investigated. The results demonstrated that all nanocarbons can greatly boost the photocatalytic H2-evolution activity over the g-C3N4/ NiS hybrids. More interestingly, it was also revealed that the amorphous AB and CB as solid-state electron mediators could lead to much larger enhancements in the H2-evolution activity of the binary g-C3N4/NiS composite photocatalysts, as compared to the highly crystallized G, CNTs, and rGO, under visible-light irradiation from an aqueous solution containing triethanolamine (TEOA). A possible photocatalytic enhancement mechanism for the improved photocatalytic activity of ternary g-C3N4/carbon/NiS photocatalysts was also proposed. It is believed that the activity enhancement can be attributed to the enhanced charge separation and the improved H2-evolution and TEOA oxidation kinetics. It is hoped that the combination of trifunctional nanocarbon materials and earth-abundant co-catalysts could provide a general strategy to enhance the photocatalytic H2 evolution over different semiconductors under visible-light illumination.

2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalysts. All chemicals were of analytical grade and used without any further purification. The pure g-C3N4 photocatalyst was synthesized via the pyrolysis (or thermal polycondensation) of urea under ambient pressure without the use of any additives, according to the previously reported procedure.61,62 In a typical synthesis, 10 g of urea was put in a crucible with a cover and heated under static air a muffle furnace at 550 °C for 4 h with a ramping rate of 5 °C min−1. After cooling to room temperature, g-C3N4 was obtained in a powder form. The binary g-C3N4/0.5%CB (weight ratio) hybrid was synthesized by a simple sonochemical approach. A typical experimental procedure was as follows: 3 g of g-C3N4 and 15 mg of carbon black were dissolved in 100 mL of absolute ethyl alcohol. The suspension was ultrasonicated for approximately 2 h. The resultant product was filtered and dried at 80 °C in an oven overnight. The binary g-C3N4/NiS was 2225


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Figure 1. XRD patterns of (a) six photocatalysts and (b) five carbon materials. 2.4. Photoelectrochemical Measurements. The working electrodes were prepared as follows: 5 mg of photocatalyst powder was added into 2 mL of ethanol make a slurry, and the powders were dispersed using ultrasonication. Here, 500 uL of the solution was injected onto a 2 cm × 3.5 cm fluorine-doped tin-oxide (FTO) glass substrate by a drop casting method. The resulting electrodes were then dried in an oven and calcined at 150 °C for 1 h in a N2 gas flow. Transient photocurrent measurements were performed on an electrochemical analyzer (CHI660E, CHI Shanghai, Inc.) in a standard three-electrode configuration, using the as-prepared working electrodes as the working electrodes, a Pt wire as the counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. A 300 W Xe arc lamp with a sharp cut-off filter served as a visible-light source (λ ≥ 420 nm). The electrochemical impedance spectroscopy (EIS) of the above-mentioned working electrodes in a three-electrode system were also recorded via a computer controlled IM6e impedance measurement unit (Zahner Elektrik, Germany) over a frequency range of 0.01−105 Hz with an ac amplitude of 2 mV in the dark. Also, 0.1 M Na2SO4 aqueous solutions were used as the electrolyte. The same electrochemical system was also used to determine the flat-band potential of the working electrodes by the Mott−Schottky (MS) method. The measurements were performed in darkness by scanning the electrodepotential from −0.5 to 1.0 V at a scan rate of 25 mV/s, and the impedance-potential characteristics were recorded at a frequency of 1 kHz. 2.5. Electrocatalytic Hydrogen Evolution. The electrocatalytic hydrogen evolution was tested using a three-electrode cell. Linear sweep voltammetry with a 5 mV−1 scan rate was performed in a 0.5 M H2SO4 electrolyte solution using a Pt plate as the counter electrode and Ag/AgCl (saturated KCl) as a reference electrode. The working electrodes were prepared as follows: 6 mg of photocatalyst power was ultrasonically dispersed in 2 mL of deionized water (for >2 h) and then deposited on a glassy carbon electrode with 3 μL of the solution. After drying, 3 μL of 0.5% Nafion solution (contain 10 vol % ethanol) was then added on top of the catalyst layer.

prepared via the direct precipitation method at room temperature, using Ni(NO3)2 and Na2S as the precursors for NiS. Following the same procedure described above, the ternary g-C3N4/CB/NiS hybrid was prepared using Ni(NO3)2 and Na2S as the precursors for NiS. In a typical synthesis, 600 mg of binary g-C3N4/CB was dispersed in 40 mL of deionized water by ultrasound, and then, 3.30 mL of 0.02 M Ni(NO3)2 solution was dropped in the dispersion. The suspension was ultrasonicated for 30 min, and then 3 mL of 0.05 M Na2S solution was dropped into the above mixed solution. After 1 h stirring at room temperature, the powder samples were filtered, rinsed with distilled water, and dried at 80 °C for 24 h. The obtained g-C3N4/CB/NiS composites with 0.5 wt % of carbon black and 1.0 wt % of NiS were denoted as g-C3N4-0.5% CB-1.0%NiS. The other ternary composites with percentages of G, CNTs, rGO, and AB at 0.5 wt % and with percentages of NiS at 1.0 wt % were prepared according to the above method and were denoted as g-C3N4-0.5%G-1.0%NiS, g-C3N4-0.5%CNT-1.0%NiS, g-C3N4-0.5% rGO-1.0%NiS, and g-C3N4-0.5%AB-1.0%NiS, respectively. 2.2. Characterization. The X-ray diffraction (XRD) patterns were obtained at room temperature using a MSAL-XD2 diffractometer with Cu Kα radiation (operated at 36 kV and 30 mA, λ = 0.15406 nm). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were performed on a JEM-2100HR (200 kV, Japan), using an accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) was performed with a VG ESCALAB250 surface analysis system using a monochromatized Al Kα X-ray source (300 W, 5 mA, and 15 kV). The base pressure was about 3 × 10−9 mbar. The UV−vis spectroscopy in the 200−800 nm was measured with a Daojin UV-2550PC diffuse reflectance spectroscope. Nitrogen adsorption−desorption isotherms were measured on a Gemini-2360 analyzer (Micromeritics Co., U.S.A.) at 77 K. The Brunauer−Emmett−Teller (BET) method was used to determine the specific surface area. The pore-size distributions were derived from the desorption branches of the isotherms using the Barrett−Joyner− Halenda (BJH) method. Raman spectra were recorded at room temperature using a Renishaw InVia micro-Raman spectrometer with laser excitation at 785 nm. The photoluminescence (PL) spectra were measured using a LS 50B (PerkinElmer, Inc., U.S.A.) with an excitation wavelength of 385 nm at room temperature. 2.3. Photocatalytic Reaction Procedures. Photocatalytic water splitting was carried out in a LabSolar H2 photocatalytic hydrogen evolution system (Perfectlight, Beijing) including a 300 W Xe lamp (PLS-SXE300, Beijing Trusttech). In a typical reaction, 50 mg of powder sample was dispersed in a Pyrex glass reactor containing the mixed solution of 85 mL water and 15 mL TEOA. Then, the system was sealed and vacuumized to keep the pressure at −0.1 MPa. Afterward, a circular cooling water system was turned on, and the reactor was irradiated with Xe lamp (300 W) under magnetic stirring. The gases evolved were analyzed online with a gas chromatograph (GC-7900, TCD, with N2 as carrier gas) after 1 h of illumination. The reaction was continued for 3 h. A cyclic experiment was carried out to investigate the photocatalytic stability of the g-C3N4/AB/NiS composite. After 3 h of reaction, H2 produced was evacuated and then run for another 3 h.

3. RESULTS AND DISCUSSION 3.1. Phase Structure, Morphology, and Composition. The crystal phase structure of the as-prepared hybrid photocatalysts was first studied by powder XRD measurements. Figure 1a displays the XRD patterns of the binary g-C3N4/NiS and the ternary g-C3N4/carbon/NiS hybrid photocatalysts. As shown in Figure 1a, all obtained powder samples exhibited two distinct diffraction peaks centered at around 27.4° and 13.1°, which are consistent with the previously reported values for the hexagonal phase of polymeric g-C3N4 (JCPDS #87-1526), confirming the formation of the graphitic-like packing layer structure.17,27 It is known that the bulk g-C3N4 stacks like graphite with 2D nanosheets, which consist of tri-s-triazines interconnected via tertiary amines.18 The strong diffraction peak at 2θ values of 27.4° typically corresponds to the (002) crystal plane of g-C3N4 with interplanar distance of 0.326 nm, which is well known for the melon networks.61 Another weak 2226


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Figure 2. TEM images of (a) g-C3N4-1.0%NiS, (b) g-C3N4-0.5%G-1.0%NiS, (c) g-C3N4-0.5%CNT-1.0%NiS, (d) g-C3N4-0.5%rGO-1.0%NiS, (e) g-C3N4-0.5%AB-1.0%NiS, and (f) g-C3N4-0.5%CB-1.0%NiS.

Figure 3. TEM (a−-d) and HRTEM (e) images, and the corresponding EDX elemental mapping images (f−j) of the ternary g-C3N4-0.5%CB-1.0% NiS nanohybrids.

due to their low concentration and highly selective dispersion of carbon and NiS species in the composites. Moreover, it is shown in Figure 1b that graphite and carbon nanotubes exhibit a distinct diffraction peak at 26.3°, which can be attributed to the (002) plane of its hexagonal graphitic structure,65 while the broad diffraction peak of CB or AB around at 25° is indicative of an amorphous carbon material.48 The pattern of rGO displayed a broad diffraction peak at around 24.5°, corresponding to the (002) interlayer d-spacing of 0.388 nm.66 All results confirm that the loading of nanocarbons and NiS nanoparticles has no significant influence on the crystalline structure of g-C3N4. The morphology and nanostructures of the as-prepared photocatalysts were further identified by TEM measurements. Figure 2 shows the TEM images of the as-prepared samples with different nanocarbons. Figure 2a displays the TEM image of the g-C3N4-1.0%NiS photocatalyst, confirming that small NiS nanoparticles with an average diameter of 420 nm) are displayed in Figure 9B. As observed in Figure 9B, it is obvious that the ternary composite photocatalysts exhibit higher photocurrent density than the binary composite photocatalysts, indicating that the metal-free carbon materials as co-catalysts could more efficiently boost the interfacial charge transfer. The enhanced photocurrent over ternary composites implies more efficient separation and transfer of photoexcited electron−hole pairs, which is greatly beneficial for the photoactivity enhancements. More importantly, g-C3N4/CB/NiS exhibits the highest photocurrent density among all ternary samples, indicating the best charge separation and transfer caused by the introduction 2231


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Figure 10. Polarization curves of NiS and different photocatalysts (A) and nanocarbons (B) were measured in 0.5 M H2SO4 acidic solution.

sample (−0.796 V vs NHE, in 0.5 M H2SO4 acidic solution), indicating that amorphous CB and AB as interfacial mediators are better than rGO for effectively improving the H2-evolution kinetics over the g-C3N4/NiS hybrids due to their higher electrical conductivity. Meanwhile, Figure 10B shows the electrocatalytic H2-evolution activity for five types of nanocarbons. As observed in Figure 10B, it is clear that these five carbons could also act as the electrocatalytic H2-evolution active sites, despite that their electrocatalytic activities (or onset potentials) are obviously smaller than those of the famous NiS co-catalysts. Further observation demonstrates that the electrocatalytic H2-evolution activities of these five carbons decrease in the following order: CNT > rGO > AB ≈ CB > G. Through a more careful and deliberate comparison between Figure 10A and B, it could be revealed that the nanocarbons with higher electrical conductivity and weaker electrocatalytic H2-evolution activities, such as AB and CB, are more suitable for being utilized as interfacial mediators to improve the H2-evolution activity of NiS co-catalysts. In other words, the nanocarbons with higher conductivity and weaker electrocatalytic H2-evolution activities as interfacial mediators could more efficiently collect and enrich the photogenerated electrons from g-C3N4 semicondcutor, which could be further utilized to significantly boost the electroncatlytic H2 evolution over the adjacent NiS co-catalysts. The weaker electrocatalytic H2-evolution activities of nanocarbons could effectively reduce the loss of collected electrons, thus maximizing their potential utilization in driving the electrocatalytic H2 evolution over more active NiS co-catalysts. As a result, the excellent synergistic effects of NiS and nanocarbons achieve the significantly enhanced charge separation and photocatalytic H2 evolution. In order to better understand the intrinsic electronic properties of the film electrode in contact with the electrolyte solution, the differences in the flat-band positions of the g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS film electrodes were examined by using the MS analysis, which is based on the assumption that the capacitance of the space charge layer is much smaller than that of the Helmholtz layer.87 Figure 11 shows MS plots, 1/C2 versus E, for g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS. The positive slope of the straight lines indicates that g-C3N4 is an n-type semiconductor, whose lowest potential of CB can be very close to the flat-band potential. The flat band potential can be obtained from the intercept of the tangents of linear potential curves on the horizontal axis, which can be approximately estimated to be −1.12 and −1.05 V versus RHE for g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS, respectively. Clearly, g-C3N4-0.5% CB-1.0%NiS displays a positive shift of the flat-band potential as compared with that of g-C3N4-1.0%NiS. According to the value of the band gap, it has been demonstrated that

of CB, which are supported by the highest photocatalytic H2-evolution activity. To further confirm the enhanced charge separation rate, the EIS Nyquist plot of all samples in the dark were also performed, which is one of the most powerful techniques to study the interfacial charge transfer and recombination rates occurring in the three-electrode systems.85,86 The EIS Nyquist plots of six different samples are shown in Figure 9C. As displayed in Figure 9C, the photocatalysts exhibit semicircles in the middlefrequency region, and the arc radius on the Nyquist plot of ternary hybrids samples are smaller than that of g-C3N4-1.0% NiS, suggesting that a slower recombination and a more efficient separation of photogenerated electron−hole pairs for them. Thus, it is clear that G, CNT, rGO, AB, and CB in the ternary hybrids could lead to accelerated charge transport and separation, thus resulting in significantly improved photocatalytic H2-production activity. In addition, it is also observed that ternary g-C3N4-0.5%AB-1.0%NiS and g-C3N4-0.5%CB1.0%NiS exhibit a smaller arc radius than the other three kinds of ternary hybrids, which are in good agreement with the highest H2-evolution activity, further confirming the outstanding roles of AB and CB in improving charge separation and transfer. Obviously, the improvements of charge-separation performances for these nanocarbons increased in the following order: G < CNT < rGO < AB ≈ CB. In a word, the similar results from transient photocurrent responses and PL and EIS spectra strongly indicated that the amorphous semimetals CB and AB are much better than rGO, CNT, and G for applications in the ternary composite photocatalysts, due to much higher electric conductivity and smaller particle sizes. 3.5. Photocatalytic Mechanism. It is known that the photogenerated electrons and holes cause reduction and oxidation reactions on surface active sites, respectively, which are similar to those in electrolysis.2,27 To further identify the key roles of NiS and nano carbon co-catalysts in improving the H2 evolution during the photocatalytic process, the polarization curves of NiS, different photocatalysts, and nanocarbons were also performed. Figure 10A shows the electrocatalytic H2-evolution activity for NiS and four kinds of nanocomposites using a standard three-electrode configuration in 0.5 M H2SO4 acidic solution. Clearly, the binary samples exhibited the proper onset potential for H2 evolution (−0.901 V vs NHE, in 0.5 M H2SO4 acidic solution, obtained by the intercept of the tangents on horizontal axis), indicating that NiS plays an important role in acting as active sites to decrease the onset potential and improve the kinetics for H2 evolution.27 More importantly, it is noteworthy from Figure 10A that the onset potentials of g-C 3 N 4 -0.5%CB-1.0%NiS and g-C 3 N 4 -0.5%AB-1.0%NiS (−0.698 and −0.704 V vs NHE, in 0.5 M H2SO4 acidic solution) are slightly smaller than that of the g-C3N4-0.5%rGO-1.0%NiS 2232


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of g-C3N4 could be excited and injected into its conduction band to drive the H2 evolution reaction, leaving the holes in the valence band for TEOA oxidation. However, the photogenerated electron−hole pairs will quickly recombine at the bulk and surface of g-C3N4 without co-catalyst loading, thus leading to its poor H2-evolution activity. Previous studies have demonstrated that the NiS co-catalysts could act as active sites to promote charge separation and decrease the overpotential of H2-evolution reaction on g-C3N4 semiconductor photocatalysts, thus achieving the significantly enhanced H2-evolution activity.27,42,43 Unfortunately, the poor electrical conductivity and P-type character of NiS co-catalysts are unfavorable for accepting and storing the electrons to drive the H2-evolution reaction. On the contrary, when inserting the carbon layer between the g-C3N4 photocatalyst and NiS co-catalysts, the photoinduced electrons in the conduction band of g-C3N4 can rapidly transfer to carbon materials due to their excellent electronic conductivity, which could hinder the fast charge recombination.40,44 More importantly, the enriched photoinduced electrons on the surface of carbon materials could accelerate the photocatalytic H2 evolution on the NiS co-catalysts as active sites. Additionally, the loaded nanocarbons can also slightly increase the oxidation ability of TEOA driven by photogenerated holes, thus leading to the improved TEOAoxidation kinetics.87,89 In a word, the boosted photocatalytic H2 evolution over gC3N4/NiS photocatalysts could be well supported by the trifunctional roles of nanocarbons: acting as effective H2-evolution co-catalysts and conductive electron bridges and increasing the oxidation ability of TEOA driven by photogenerated holes due to the lowered valance band position. Accordingly, it is clear that the combined effects of improved charge transfer and separation and H2-evolution kinetics and TEOA-oxidation capacity could directly achieve the significantly enhanced co-catalyst-assisted photocatalytic H2 evolution activity over the ternary multiheterostructured hybrid photocatalysts.

Figure 11. MS plots of two different photocatalysts film electrodes. MS plots were obtained at a frequency of 1 kHz in an aqueous solution of Na2SO4 (0.1 M).

g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS exhibit a similar band gap of ca. 2.71 eV. Thus, the calculated positions of the valence band maximum (VBM) are 1.59 and 1.66 V versus RHE for g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS, respectively. The VBM of g-C3 N 4 -0.5%CB-1.0%NiS is lower than that of g-C3N4-1.0%NiS by 0.07 V, suggesting that g-C3N4-0.5%CB-1.0%NiS has a stronger oxidation ability for TEOA theoretically. More recently, the enhanced H2 evolution has also been realized by improved water oxidation kinetics.88 It was believed that the metal-free carbon nanodots could promote the decomposition of H2O2 generated on the proximity active sites on the surface of g-C3N4, thus achieving the highest overall water splitting so far.88 According to the photocatalytic mechanism, the improved TEOA oxidation ability will further accelerate the separation efficiency of electron−hole pairs, thus directly contributing to the enhancement of photocatalytic H2-evolution activity. The results were also well supported by the improved photocurrent and EIS and PL spectra. On the basis of the above-mentioned observations and results, it is clear that the nanocarbons are typically trifunctional materials in these interestingly ternary systems. On the one hand, nanocarbons could obviously serve as H2-evolution co-catalysts due to their much lower electrocatalytic H2-evolution overpotentials than that of pure g-C3N4 nanosheets. At this point, the nanocarbons themselves could play an important role in increasing the surface H2-evolution active sites over g-C3N4 nanosheets. On the other hand, nanocarbons could also act as conductive bridges to significantly boost the H2 evolution over NiS co-catalysts. The possible mechanisms for separation and transport of the electron−hole pairs in the binary g-C3N4/NiS and ternary g-C3N4/carbon/NiS composites under visible-light irradiation are illustrated in Scheme 2A and B, respectively. Under visible-light irradiation, the electrons in the valence band

4. CONCLUSIONS In summary, a series of semimetallic nanocarbons, including G, CNT, rGO, AB, and CB, have been successfully coupled into the interface between g-C3N4 and earth-abundant NiS co-catalysts using a facile precipitation method. The resulting ternary multiheterostructured g-C3N4/carbon/NiS photocatalysts exhibit excellent photocatalytic activity and stability for H2 generation from a 15 vol % TEOA aqueous solution under visible-light irradiation. The results showed that the g-C3N4-0.5%CB-1.0% NiS and g-C3N4-0.5%AB-1.0%NiS photocatalysts exhibited the highest H2-evolution rates of 366.4 and 297.7 μmol g−1 h−1,

Scheme 2. Schematic Diagram of Photocatalytic H2 Evolution over (A) Binary g-C3N4/NiS Co-Catalyst and (B) Ternary g-C3N4/Carbon/NiS Co-Catalyst Composites under Visible-Light Irradiation

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which are 3.17 and 2.57 times higher than that of g-C3N4-1.0% NiS, respectively. It is believed that the enhanced photocatalytic activity can be attributed to the combined effects of improved charge transfer and separation, accelerated H2-evolution kinetics, and enhanced oxidation ability of holes due to the lowerd valance band position. Through the thorough discussion of trifunctional roles of nanocarbons, the possible co-catalystassisted photocatalytic mechanisms have been proposed to well explain the enhanced photocatalysis in the ternary g-C3N4/ carbon/NiS composites. This work demonstrated that the amorphous semimetal CB and AB as interfacial mediators are much better than rGO, CNT, and G for enhancing the charge transfer and separation from g-C3N4 to NiS and significantly boosting the H2-evolution activity of NiS co-catalysts, which may give a new mechanistic insight into the development of multiheterostructured g-C3N4-based composite photocatalysts for various applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 85282633. Fax: +86 20 85285596. E-mail: [email protected] (X. Li). ORCID

Xin Li: 0000-0002-4842-5054 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National Natural Science Foundation of China (51672089), the industry and research collaborative innovation major projects of Guangzhou (201508020098), and the State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (2015-KF-7).

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