NiS Multiple

Subscriber access provided by UNIV OF NEWCASTLE

Article

Fabricating the robust g-C3N4 nanosheets/carbons/NiS multiple heterojunctions for enhanced photocatalytic H2 generation: An Insight into the tri-functional roles of nanocarbons Jiuqing Wen, Jun Xie, Zhuohong Yang, Rongchen Shen, Huiyi Li, Xingyi Luo, Xiaobo Chen, and Xin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02490 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Fabricating

the

robust

g-C3N4

nanosheets/carbons/NiS

multiple heterojunctions for enhanced photocatalytic H2 generation: An Insight into the tri-functional roles of nanocarbons

Jiuqing Wen,a,b,cJun Xie,a,b,c Zhuohong Yang,a, c Rongchen Shen,a,b,c Huiyi Li,a,b,c XingYi Luo,a, c Xiaobo Chen,d Xin Lia,b,c *

a

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, PR

China b

College of Forestry and Landscape Architecture, South China Agricultural University,

Guangzhou 510642, PR China c

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, PR China. d

Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, 64110, USA.

*

Corresponding author at: College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China. Tel.: +86 20 85282633; fax: +86 20 85285596. E-mail address: [email protected] (X. Li). 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

ABSTRACT In this work, the 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 tri-functional roles in boosting the photocatalytic H2 evolution over g-C3N4, which can not only act as effective H2-evolution cocatalysts, but also can serve as conductive electron bridges to collect photo-generated electrons and boost the H2evolution kinetics over the NiS cocatalysts. More interestingly, the nanocarbons can also result in the downshift of valance band of g-C3N4, thus facilitating the fast oxidation of triethanolamine and charge-carrier separation. Particularly, in all five ternary multi-heterostructured systems, the gC3N4-0.5%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. It is believed that the nanocarbons mainly act as conductive electron bridges in boosting the H2-evolution activity of multi-heterostructured gC3N4/carbon/NiS composite semiconductors, instead of as H2-evolution cocatalysts. 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 cocatalysts for maximizing the H2 generation. This work may give a new mechanistic insight into the development of multi-heterostructured g-C3N4-based composite photocatalysts using

the

combination

of

tri-functional

nanocarbon

bridges

and

earth-abundant

cocatalysts/semiconductors for various photocatalytic applications. KEYWORDS: Photocatalytic Hydrogen Evolution, NiS co-catalysts, g-C3N4, tri-functional nanocarbons, graphene

2


Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 significant amounts of research interests 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 gC3N4, with a medium band gap of 2.7 eV, has been identified as one of the exciting visible-lightdriven photocatalysts for solar fuel production,11-16 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) 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 -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 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 the efficient photocatalysis.2,27,28 However, 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 cocatalysts 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 electro-catalysts, such as the non-precious metal-based, metal-free and their hybrid materials, have been found to be extremely important for achieving robust electrocatalytic hydrogen generation.29-34 More importantly, these electro-catalysts could be selectively deposited onto various nanostructured heterogeneous semiconductors as waterreduction cocatalysts 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 electro-catalysts have been employed as cocatalysts to enhance the photocatalytic H2 generation over g-C3N4.55 However, for these earth-abundant cocatalysts, 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 photo-generated 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 photo-excited electrons from the conduction band n-type g-C3N4 to the p-type cocatalysts. Therefore, to further enhance the activity and stability over the gC3N4/noble metal-free cocatalysts 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 cocatalysts. More interestingly, despite the significant advances in the 4


Page 4 of 40

Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


promoted photocatalytic H2 evolution over CdS and TiO2 semiconductors through combing the rGO, other various kinds of earth-abundant cocatalysts such as MoS2,56,57 WS2,58 Ni(OH)259 and NiS,60 limited studies have been reported on the photocatalytic H2 evolution over the ternary hybrids of g-C3N4/conductive carbons/earth-abundant cocatalysts.40,44 Thus, it would be of great interest to enhance the photocatalytic H2 evolution over the ternary g-C3N4/carbon/earth-abundant cocatalysts 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 multi-heterostructured hybrids with different carbon bridges 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 cocatalysts and electron transfer bridges for enhancing the photocatalytic H2-evolution activity over the g-C3N4/NiS hybrids. The ternary hybrids will be synthetized by a simple and facile wetchemistry method: ultrasonic dispersion treatments and the subsequent precipitation process (as shown in Scheme 1). The tri-functional roles of nanocarbons in boosting the photocatalytic H2 evolution over g-C3N4 will be 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 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

light irradiation from an aqueous solution containing triethanolamine (TEOA). A possible photocatalytic enhancement mechanism for the improved photocatalytic activity of ternary gC3N4/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 tri-functional nano carbon materials and earth-abundant cocatalysts could provide a general strategy to enhance the photocatalytic H2 evolution over different semiconductors under visible light illumination.

Scheme 1. Schematic illustration for the fabrication of ternary g-C3N4/carbon/NiS composites. 2. Experimental Section 2.1. Preparation of photocatalysts All chemicals were of analytical grade and used without any further purification. The pure gC3N4 photocatalyst was synthesized via the pyrolysis (or thermal poly-condensation) of urea under ambient pressure without the use of any additives, according to the previously reported 6


Page 6 of 40

Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 carbon black were dissolved in 100 mL of absolute ethyl alcohol. The suspension was ultrasonicated for approximately 2 hours. The resultant product was filtered and dried at 80 °C in an oven overnight. The binary g-C3N4/NiS was 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, 600mg of binary gC3N4/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 hour 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, AB at 0.5 wt% and with percentages of NiS at 1.0 wt% were prepared according above method and were denoted as gC3N4-0.5%G-1.0%NiS,

g-C3N4-0.5%CNT-1.0%NiS,

g-C3N4-0.5%rGO-

0.5%AB-1.0%NiS , respectively. 2.2. Characterization 7


1.0%NiS,

g-C3N4-


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 40

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=0.15406nm). 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 Spectroscopy. Nitrogen adsorption– desorption isotherms were measured on a Gemini-2360 analyzer (Micromeritics Co., USA) 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

785nm.

The

photoluminescence (PL) spectra were measured using a LS 50B (Perkin Elmer, Inc., USA) 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 as -0.1 MPa. Afterwards, 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 on line with a gas chromatograph (GC-7900, TCD, with N2 as carrier gas) after 1 h of 8


Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. 2.4 Photoelectrochemical measurements experiments. 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. 500 uL of the solution was injected onto a 2×3.5 cm2 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 asprepared 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 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. 0.1 M Na2SO4 aqueous solution 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 form −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 The 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 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 were ultrasonically dispersed in 2 mL of deionized water (for > 2 h) and then deposited on 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.

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 by 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 peak at 13.1° can be assigned to (100) crystal plane of g-C3N4, which corresponds to the in-planar ordering of tri-s-triazine units with a period of 0.675 nm.40 Notably, two diffraction peaks of g-C3N4 nanosheets are still relatively broad, implying their low degree of crystallinity by the conventional polycondensation of nitrogen-rich molecules as precursors at high temperatures (400–600 °C) in air, due to the existence of structural defects. In this regard, fabricating the highly crystalline g-C3N4 nanosheets with reduced structural 10


Page 10 of 40

Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


defects through different strategies, such as microwave-assisted thermolysis63 and the ionothermal synthesis (molten salt strategy),64 seem to be more promising in developing the highly efficient robust g-C3N4-based materials for various kinds of photocatalytic applications. In addition, after the deposition of nanocarbons and NiS nanoparticles on the surface of g-C3N4, no additional diffraction peaks for them can be detected. Clearly, the unchanged features of the XRD patterns further indicated that the original atomic structure is largely retained, which may be due to their low concentration and highly selective dispersion of carbon and NiS species in the composites. Moreover, it can be seen from Figure 1b that graphite and carbon nanotube 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 confirmed that the loading of nanocarbons and NiS nanoparticles has no significant influence on the crystalline structure of g-C3N4.

Figure 1. XRD patterns of the (a) six photocatalysts and (b) five carbon materials. The morphology and nanostructures of the as-prepared photocatalysts were further identified by 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

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 420 nm) were display in Figure 9B. As observed in Figure 9B, it is obvious that the ternary composite photocatalysts exhibit higher photocurrent density than binary composite photocatalyst, indicating that the metal-free carbon materials cocatalysts could more efficiently boost the interfacial charge transfer. The enhanced photocurrent over ternary composites implies more efficient separation and transfer of photo-excited electron–hole pairs, which is greatly beneficial for the photoactivity enhancements. More importantly, the g-C3N4/CB/NiS exhibits the highest photocurrent density 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

among all ternary samples, indicating the best charge separation and transfer caused by the introduction 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 middle-frequency 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 the G, CNT, rGO, AB and CB in the ternary hybrids could lead to the accelerated charge transport and separation, thus resulting in the significantly improved photocatalytic H2-production activity. In addition, it is also observed that the ternary g-C3N40.5%AB- 1.0%NiS and g-C3N4-0.5%CB-1.0%NiS exhibit the smaller arc radius than 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 the 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 semimetal CB and AB are much better than the rGO, CNT and G for the applications in the ternary composite photocatalysts, due to super higher electric conductivity and smaller particle sizes. 3.5 Photocatalytic mechanism It is known that the photogenerated electrons and holes cause the reduction and oxidation reactions on surface active sites, respectively, which are similar to those in electrolysis.2,27 To 26


Page 26 of 40

Page 27 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


further identify the key roles of NiS and nano carbon cocatalysts 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-C3N4-0.5%CB-1.0%NiS and g-C3N40.5%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 sample (-0.796 V vs NHE, in 0.5 M H2SO4 acidic solution), indicating that the 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 their electrocatalytic activities (or onset potentials) are obviously smaller than that of the famous NiS cocatalysts. 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 Figure 10B, 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 cocatalysts. In other words, the nanocarbons with higher conductivity and weaker electrocatalytic H2-evolution activities as interfacial mediators could 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

more efficiently collect and enrich the photo-generated electrons from g-C3N4 semicondcutor, which could be further utilized to significantly boost the electroncatlytic H2 evolution over the adjacent NiS cocatalysts. The weaker electrocatalytic H2-evolution activities of nanocarbons could effectively reduce the loss of collected electrons, thus maximizing their potential utilization in driven the electrocatalytic H2 evolution over more active NiS cocatalysts. As a result, the excellent synergistic effects of NiS and nanocarbons achieve the significantly enhanced charge separation and photocatalytic H2 evolution.

Figure 10. Polarization curves of NiS and different photocatalysts (A) and nanocarbons (B) were measured in 0.5 M H2SO4 acidic solution. 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 g-C3N4-1.0%NiS and gC3N4-0.5%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 the g-C3N4-1.0%NiS and g-C3N40.5%CB-1.0%NiS. The positive slope of the straight lines indicates that g-C3N4 is an n-type semiconductor, whose lowest potential of the CB can be very close to the flat-band potential. The 28


Page 28 of 40

Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


flat band potential can be obtained from the intercept of the tangents of linear potential curves on horizontal axis, which can be approximately estimated to be −1.01 and −0.96 V versus RHE for the g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS, respectively. Clearly, the g-C3N4-0.5%CB1.0%NiS displays a positive shift of the flat-band potential as compared with that of g-C3N41.0%NiS. According to the value of band gap, it has been demonstrated that the g-C3N4-1.0%NiS and g-C3N4-0.5%CB-1.0%NiS exhibit the similar band gap of ca. 2.71 eV. Thus, the calculated positions of valence band maximum (VBM) are 1.70 and 1.75 V versus RHE for the g-C3N41.0%NiS and g-C3N4-0.5%CB-1.0%NiS, respectively. The VBM of g-C3N4-0.5%CB-1.0%NiS is lower than that of g-C3N4-1.0%NiS by 0.05 V, suggesting that the g-C3N4-0.5%CB-1.0%NiS has stronger oxidation ability for TEOA theoretically. More recently, the enhanced H2 evolution has also been realized by the 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, EIS and PL spectra.

29



3.5

C-2·109 (cm4F-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.0

g-C3N4-1.0% NiS g-C3N4-0.5% CB-1.0% NiS

2.5 2.0 1.5

-1.25 V 1.0 -1.32 V 0.5 0.0 -1.5

-1.2 -0.9 -0.6 -0.3 0.0 Potential (V vs Ag/AgCl)

0.3

Figure 11. MS plots of two different photocatalysts film electrodes. The MS plots were obtained at a frequency of 1 kHz in an aqueous solution of Na2SO4 (0.1 M). Based on above-mentioned observations and results, it is clear that the nanocarbons are typically tri-functional materials in these interestingly ternary systems. On the one hand, nanocarbons could obviously serve as H2-evolution cocatalysts due to their much lower elcctrocatalytic 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 cocatalysts, respectively. 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 2B, respectively. Under visible light irradiation, the electrons in the valence band of g-C3N4 could be excited and injected 30


Page 30 of 40

Page 31 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


into its conduction band to drive the H2 evolution reaction, leaving the holes in the valence band for TEOA oxidation. However, the photo-generated 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 cocatalysts could act as active sites to promote the 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 cocatalysts are unfavorable for accepting and storing the electrons to drive the H2-evolution reaction. On the contrary, when inserting carbon layer between g-C3N4 photocatalyst and NiS cocatalysts, the photo-induced 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 photo-induced electrons on the surface of carbon materials could accelerate the photocatalytic H2 evolution on the NiS cocatalysts as active sites. Additionally, the loaded nanocarbons can also slightly increase the oxidation ability of TEOA driven by photo-generated holes, thus leading to the improved TEOA-oxidation kinetics.87,89

Scheme 2. The schematic diagram of photocatalytic H2 evolution over (A) binary g-C3N4/NiS cocatalyst, and (B) ternary g-C3N4/carbon/NiS co-catalyst composites under visible light irradiation. 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

In a word, the boosted photocatalytic H2 evolution over g-C3N4/NiS photocatalysts could be well supported by the tri-functional roles of nanocarbons: acting as effective H2-evolution cocatalysts and conductive electron bridges, and increasing the oxidation ability of TEOA driven by photo-generated holes due to the lowed conduction 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

multi-heterostructured

hybrid

photocatalysts. 4. Conclusions In summary, a series of semi-metallic nano-carbons, including G, CNT, rGO, AB and CB, have been successfully coupled into the interface between g-C3N4 and earth-abundant NiS cocatalysts using a facile precipitation method. The resulting ternary multi-heterostructured gC3N4/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, 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 the enhanced oxidation ability of holes due to the lowed conduction band position. Through the thorough discussion of tri-functional roles of nanocarbons and NiS, the possible cocatalyst-assisted 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


32

Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mediators are much better than the 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 cocatalysts/semiconductors, which may give a new mechanistic insight into the development of multi-heterostructured g-C3N4-based composite photocatalysts for various applications. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Li). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 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).

REFERENCES (1) Turner, J. A. A Realizable Renewable Energy Future. Science 1999, 285, 687-689. (2) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering heterogeneous semiconductors for solar water splitting. J. Mater. Chem. A 2015, 3, 2485-2534. (3) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37-38. (4) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503-6570. (5) Chao, K.-J.; Cheng, W.-Y.; Yu, T.-H.; Lu, S.-Y. Large enhancements in hydrogen


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

production of TiO2 through a simple carbon decoration. carbon 2013, 62, 69-75. (6) Liu, J.; Xu, S.; Liu, L.; Sun, D. D. The size and dispersion effect of modified graphene oxide sheets on the photocatalytic H2 generation activity of TiO2 nanorods. carbon 2013, 60, 445-452. (7) Wang, X.; Yao, X. Photocatalytic hydrogen generation of ZnO rod-CdS/reduced graphene oxide heterostructure prepared by Pt-induced oxidation and light irradiation-assisted methods. carbon 2014, 77, 667-674. (8) Li, Q.; Li, X.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J. CdS/Graphene Nanocomposite Photocatalysts. Adv. Energy Mater. 2015, 5, 1500010-n/a. (9) Hsu, M.-H.; Chang, C.-J.; Weng, H.-T. Efficient H-2 Production Using Ag2SCoupled ZnO@ZnS Core-Shell Nanorods Decorated Metal Wire Mesh as an Immobilized Hierarchical Photocatalyst. Acs Sustain. Chem. Eng. 2016, 4, 1381-1391. (10) Zhang, L.; Jing, D.; Guo, L.; Yao, X. In Situ Photochemical Synthesis of ZnDoped Cu2O Hollow Microcubes for High Efficient Photocatalytic H-2 Production. Acs Sustain. Chem. Eng. 2014, 2, 1446-1452. (11) Xiang, Q.; Cheng, B.; Yu, J. Graphene-Based Photocatalysts for Solar-Fuel Generation. Angew. Chem. Int. Edit. 2015, 54, 11350-11366. (12) Low, J.; Yu, J.; Ho, W. Graphene-Based Photocatalysts for CO2 Reduction to Solar Fuel. J. Phys. Chem. Lett. 2015, 6, 4244-4251. (13) Ye, S.; Wang, R.; Wu, M.-Z.; Yuan, Y.-P. A review on g-C3N4 for photocatalytic water splitting and CO2 reduction. Appl. Surf. Sci. 2015, 358, 15-27. (14) Li, X.; Wen, J.; Low, J.; Fang, Y.; Yu, J. Design and fabrication of semiconductor photocatalyst for photocatalytic reduction of CO2 to solar fuel. Science China Materials 2014, 57, 70–100. (15) Cao, S.; Yu, J. g-C3N4-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2014, 5, 2101-2107. (16) Wen, J.; Xie, J.; Chen, X.; Li, X. A review on g-C3N4-based photocatalysts. Appl. Surf. Sci. 2017, 391, 72-123. (17) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76-80. (18) Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150-2176. (19) Zhang, J.; Sun, J.; Maeda, K.; Domen, K.; Liu, P.; Antonietti, M.; Fu, X.; Wang, X. Sulfur-mediated synthesis of carbon nitride: Band-gap engineering and improved functions for photocatalysis. Energy Environ. Sci. 2011, 4, 675-678. (20) Chuang, P.-K.; Wu, K.-H.; Yeh, T.-F.; Teng, H. Extending the pi-Conjugation of gC3N4 by Incorporating Aromatic Carbon for Photocatalytic H2 Evolution from Aqueous Solution. Acs Sustain. Chem. Eng. 2016, 4, 5989-5997. (21) Zhang, X.; Peng, B.; Zhang, S.; Peng, T. Robust Wide Visible-Light-Responsive Photoactivity for H2 Production over a Polymer/Polymer Heterojunction Photocatalyst: The Significance of Sacrificial Reagent. Acs Sustain. Chem. Eng. 2015, 3, 1501-1509. (22) Lang, J.; Liu, M.; Su, Y.; Yan, L.; Wang, X. Fabrication of the heterostructured CsTaWO6/Au/g-C3N4 hybrid photocatalyst with enhanced performance of photocatalytic hydrogen production from water. Appl. Surf. Sci. 2015, 358, 252-260. (23) Zhang, J.; Huang, F. Enhanced visible light photocatalytic H2 production activity


34

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of g-C3N4 via carbon fiber. Appl. Surf. Sci. 2015, 358, 287-295. (24) Han, C.; Wu, L.; Ge, L.; Li, Y.; Zhao, Z. AuPd bimetallic nanoparticles decorated graphitic carbon nitride for highly efficient reduction of water to H2 under visible light irradiation. carbon 2015, 92, 31-40. (25) Li, X.; Yu, J.; Jaroniec, M. Hierarchical photocatalysts. Chem. Soc. Rev. 2016, 45, 2603-2636. (26) Lin, Z.; Lin, L.; Wang, X. Thermal nitridation of triazine motifs to heptazinebased carbon nitride frameworks for use in visible light photocatalysis. Chinese. J. Catal. 2015, 36, 2089-2094. (27) Yuan, J.; Wen, J.; Zhong, Y.; Li, X.; Fang, Y.; Zhang, S.; Liu, W. Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/g-C3N4 heterojunctions. J. Mater. Chem. A 2015, 3, 18244-18255. (28) Yuan, Y.-P.; Ruan, L.-W.; Barber, J.; Joachim Loo, S. C.; Xue, C. Heteronanostructured suspended photocatalysts for solar-to-fuel conversion. Energy Environ. Sci. 2014, 7, 3934-3951. (29) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@NGraphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. Acs Nano 2015, 9, 931-940. (30) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (31) Han, A.; Jin, S.; Chen, H.; Ji, H.; Sun, Z.; Du, P. A robust hydrogen evolution catalyst based on crystalline nickel phosphide nanoflakes on three-dimensional graphene/nickel foam: high performance for electrocatalytic hydrogen production from pH 0-14. J. Mater. Chem. A 2015, 3, 1941-1946. (32) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 2014, 5, 3783. (33) Li, X.; Yu, J.; Wageh, S.; Al-Ghamdi, A. A.; Xie, J. Graphene in Photocatalysis: A Review. Small. (34) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem. Int. Edit. 2015, 54, 52-65. (35) You, X.; Rong, X. Nickel-based cocatalysts for photocatalytic hydrogen production. Appl. Surf. Sci. 2015, 351, 779-793. (36) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting. Chem. Soc. Rev. 2014, 43, 7787-7812. (37) Zhou, Y.; Zhang, L.; Huang, W.; Kong, Q.; Fan, X.; Wang, M.; Shi, J. N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H-2 evolution under visible light. carbon 2016, 99, 111-117. (38) Zhu, Y.; Xu, Y.; Hou, Y.; Ding, Z.; Wang, X. Cobalt sulfide modified graphitic carbon nitride semiconductor for solar hydrogen production. Int. J. Hydrogen. Energ. 2014, 39, 11873-11879. (39) Zhou, X.; Luo, Z.; Tao, P.; Jin, B.; Wu, Z.; Huang, Y. Facile preparation and enhanced photocatalytic H2-production activity of Cu(OH)2 nanospheres modified porous gC3N4. Mater. Chem. Phys. 2014, 143, 1462-1468. (40) Zhong, Y.; Yuan, J.; Wen, J.; Li, X.; Xu, Y.; Liu, W.; Zhang, S.; Fang, Y. Earthabundant NiS co-catalyst modified metal-free mpg-C3N4/CNT nanocomposites for highly


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

efficient visible-light photocatalytic H2 evolution. Dalton Trans. 2015, 44, 18260-18269. (41) Yin, L.; Yuan, Y.-P.; Cao, S.-W.; Zhang, Z.; Xue, C. Enhanced visible-light-driven photocatalytic hydrogen generation over g-C3N4 through loading the noble metal-free NiS2 cocatalyst. Rsc. Adv. 2014, 4, 6127-6132. (42) Chen, Z.; Sun, P.; Fan, B.; Zhang, Z.; Fang, X. In Situ Template-Free IonExchange Process to Prepare Visible-Light-Active g-C3N4/NiS Hybrid Photocatalysts with Enhanced Hydrogen Evolution Activity. J. Phys. Chem. C 2014, 118, 7801-7807. (43) Hong, J.; Wang, Y.; Wang, Y.; Zhang, W.; Xu, R. Noble-Metal-Free NiS/C3N4 for Efficient Photocatalytic Hydrogen Evolution from Water. Chemsuschem 2013, 6, 2263-2268. (44) Wen, J.; Li, X.; Li, H.; Ma, S.; He, K.; Xu, Y.; Fang, Y.; Liu, W.; Gao, Q. Enhanced visible-light H2 evolution of g-C3N4 photocatalysts via the synergetic effect of amorphous NiS and cheap metal-free carbon black nanoparticles as co-catalysts. Appl. Surf. Sci. 2015, 358, 204-212. (45) Yuan, Y.-P.; Cao, S.-W.; Yin, L.-S.; Xu, L.; Xue, C. NiS2 Co-catalyst decoration on CdLa2S4 nanocrystals for efficient photocatalytic hydrogen generation under visible light irradiation. Int. J. Hydrogen. Energ. 2013, 38, 7218-7223. (46) Cao, S.-W.; Yuan, Y.-P.; Barber, J.; Loo, S. C. J.; Xue, C. Noble-metal-free gC3N4/Ni(dmgH)2 composite for efficient photocatalytic hydrogen evolution under visible light irradiation. Appl. Surf. Sci. 2014, 319, 344-349. (47) Yu, J.; Wang, S.; Cheng, B.; Lin, Z.; Huang, F. Noble metal-free Ni(OH)2-g-C3N4 composite photocatalyst with enhanced visible-light photocatalytic H-2-production activity. Catal. Sci. Technol. 2013, 3, 1782-1789. (48) Bi, G.; Wen, J.; Li, X.; Liu, W.; Xie, J.; Fang, Y.; Zhang, W. Efficient visible-light photocatalytic H2 evolution over metal-free g-C3N4 co-modified with robust acetylene black and Ni(OH)2 as dual co-catalysts. Rsc. Adv. 2016, 6, 31497-31506. (49) Zhang, G.; Li, G.; Wang, X. Surface Modification of Carbon Nitride Polymers by Core-Shell Nickel/Nickel Oxide Cocatalysts for Hydrogen Evolution Photocatalysis. Chemcatchem 2015, 7, 2864-2870. (50) Ge, L.; Han, C.; Xiao, X.; Guo, L. Synthesis and characterization of composite visible light active photocatalysts MoS2-g-C3N4 with enhanced hydrogen evolution activity. Int. J. Hydrogen. Energ. 2013, 38, 6960-6969. (51) Hou, Y.; Laursen, A. B.; Zhang, J.; Zhang, G.; Zhu, Y.; Wang, X.; Dahl, S.; Chorkendorff, I. Layered Nanojunctions for Hydrogen-Evolution Catalysis. Angew. Chem. Int. Edit. 2013, 52, 3621-3625. (52) Hou, Y.; Zhu, Y.; Xu, Y.; Wang, X. Photocatalytic hydrogen production over carbon nitride loaded with WS2 as cocatalyst under visible light. Appl. Catal. B-Environ. 2014, 156, 122-127. (53) Akple, M. S.; Low, J.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J.; Zhang, J. Enhanced visible light photocatalytic H2-production of g-C3N4/WS2 composite heterostructures. Appl. Surf. Sci. 2015, 358, 196-203. (54) Cheng, R.; Zhang, L.; Fan, X.; Wang, M.; Li, M.; Shi, J. One-step construction of FeOx modified g-C3N4 for largely enhanced visible-light photocatalytic hydrogen evolution. carbon 2016, 101, 62-70. (55) Yin, S.; Han, J.; Zhou, T.; Xu, R. Recent progress in g-C3N4 based low cost photocatalytic system: activity enhancement and emerging applications. Catal. Sci. Technol. 2015, 5, 5048-5061.


36

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Lang, D.; Shen, T.; Xiang, Q. Roles of MoS2 and Graphene as Cocatalysts in the Enhanced Visible-Light Photocatalytic H-2 Production Activity of Multiarmed CdS Nanorods. Chemcatchem 2015, 7, 943-951. (57) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575-6578. (58) Xiang, Q.; Cheng, F.; Lang, D. Hierarchical Layered WS2/Graphene-Modified CdS Nanorods for Efficient Photocatalytic Hydrogen Evolution. Chemsuschem 2016, 9, 9961002. (59) Yan, Z.; Yu, X.; Han, A.; Xu, P.; Du, P. Noble-Metal-Free Ni(OH)2-Modified CdS/Reduced Graphene Oxide Nanocomposite with Enhanced Photocatalytic Activity for Hydrogen Production under Visible Light Irradiation. J. Phys. Chem. C 2014, 118, 22896-22903. (60) Jun, Z.; Lifang, Q.; Jingrun, R.; Jiaguo, Y.; Shi Zhang, Q. Ternary NiS/ZnxCd1xS/Reduced Graphene Oxide Nanocomposites for Enhanced Solar Photocatalytic H2-Production Activity. Adv. Energy Mater. 2014, 4, 1301925 (1301926 pp.)-1301925 (1301926 pp.). (61) Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S. C. Efficient synthesis of polymeric g-C3N4 layered materials as novel efficient visible light driven photocatalysts. J. Mater. Chem. 2011, 21, 15171-15174. (62) Liu, J.; Zhang, T.; Wang, Z.; Dawson, G.; Chen, W. Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity. J. Mater. Chem. 2011, 21, 14398-14401. (63) Guo, Y.; Li, J.; Yuan, Y.; Li, L.; Zhang, M.; Zhou, C.; Lin, Z. A Rapid MicrowaveAssisted Thermolysis Route to Highly Crystalline Carbon Nitrides for Efficient Hydrogen Generation. Angewandte Chemie International Edition 2016, 55, 14693-14697. (64) Schwinghammer, K.; Mesch, M. B.; Duppel, V.; Ziegler, C.; Senker, J.; Lotsch, B. V. Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 1730-1733. (65) Chen, Y.; Li, J.; Hong, Z.; Shen, B.; Lin, B.; Gao, B. Origin of the enhanced visible-light photocatalytic activity of CNT modified g-C3N4 for H2 production. Phys. Chem. Chem. Phys. 2014, 16, 8106-8113. (66) Xie, X.; Lin, L.; Liu, R.-Y.; Jiang, Y.-F.; Zhu, Q.; Xu, A.-W. The synergistic effect of metallic molybdenum dioxide nanoparticle decorated graphene as an active electrocatalyst for an enhanced hydrogen evolution reaction. J. Mater. Chem. A 2015, 3, 8055-8061. (67) Razzaq, A.; Grimes, C. A.; In, S.-I. Facile fabrication of a noble metal-free photocatalyst: TiO2 nanotube arrays covered with reduced graphene oxide. carbon 2016, 98, 537-544. (68) Xu, T.; Zhang, L.; Cheng, H.; Zhu, Y. Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study. Applied Catalysis B: Environmental 2011, 101, 382-387. (69) Chen, Y.; Li, J.; Hong, Z.; Shen, B.; Lin, B.; Gao, B. Origin of the enhanced visible-light photocatalytic activity of CNT modified g-C3N4 for H2 production. Phys. Chem. Chem. Phys. 2014, 16, 8106-8113. (70) Zhang, Z.; Li, X.; Wang, C.; Fu, S.; Liu, Y.; Shao, C. Polyacrylonitrile and Carbon Nanofibers with Controllable Nanoporous Structures by Electrospinning. Macromol. Mater. Eng. 2009, 294, 673-678. (71) Wu, M.; Yan, J.-M.; Tang, X.-n.; Zhao, M.; Jiang, Q. Synthesis of Potassium-


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

Modified Graphitic Carbon Nitride with High Photocatalytic Activity for Hydrogen Evolution. Chemsuschem 2014, 7, 2654-2658. (72) Pawar, R. C.; Kang, S.; Ahn, S. H.; Lee, C. S. Gold nanoparticle modified graphitic carbon nitride/multi-walled carbon nanotube (g-C3N4/CNTs/Au) hybrid photocatalysts for effective water splitting and degradation. Rsc. Adv. 2015, 5, 24281-24292. (73) Zhu, B.; Xia, P.; Ho, W.; Yu, J. Isoelectric point and adsorption activity of porous g-C3N4. Appl. Surf. Sci. 2015, 344, 188-195. (74) Shi, L.; Liang, L.; Wang, F.; Liu, M.; Chen, K.; Sun, K.; Zhang, N.; Sun, J. Higher Yield Urea-Derived Polymeric Graphitic Carbon Nitride with Mesoporous Structure and Superior Visible-Light-Responsive Activity. Acs Sustain. Chem. Eng. 2015, 3, 3412-3419. (75) Akple, M. S.; Low, J.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J.; Zhang, J. Enhanced visible light photocatalytic H2-production of g-C3N4/WS2 composite heterostructures. Appl. Surf. Sci. 2015, 358, 196-203. (76) Huang, Q.; Yu, J.; Cao, S.; Cui, C.; Cheng, B. Efficient photocatalytic reduction of CO2 by amine-functionalized g-C3N4. Appl. Surf. Sci. 2015, 358, 350-355. (77) Sing, K.; Everett, D.; Haul, R.; Moscou, L.; Pierotti, R.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem 1985, 57, 603-619. (78) Li, X.; Liu, H.; Luo, D.; Li, J.; Huang, Y.; Li, H.; Fang, Y.; Xu, Y.; Zhu, L. Adsorption of CO2 on heterostructure CdS(Bi2S3)/TiO2 nanotube photocatalysts and their photocatalytic activities in the reduction of CO2 to methanol under visible light irradiation. Chem. Eng. J. 2012, 180, 151-158. (79) Li, X.; Xia, T.; Xu, C.; Murowchick, J.; Chen, X. Synthesis and photoactivity of nanostructured CdS-TiO2 composite catalysts. Catal. Today 2014, 225, 64-73. (80) Ikeda, S.; Takata, T.; Kondo, T.; Hitoki, G.; Hara, M.; N. Kondo, J.; Domen, K.; Hosono, H.; Kawazoe, H.; Tanaka, A. Mechano-catalytic overall water splitting. Chem. Commun. 1998, 2185-2186. (81) Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A. Metal-free graphitic carbon nitride as mechano-catalyst for hydrogen evolution reaction. J. Catal. 2015, 332, 149155. (82) Wang, X.; Hong, M.; Zhang, F.; Zhuang, Z.; Yu, Y. Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2 for Efficient Photocatalysis of RhB and Cr(VI) Driven by Visible Light. Acs Sustain. Chem. Eng. 2016, 4, 4055-4062. (83) Chen, D.; Wang, K.; Hong, W.; Zong, R.; Yao, W.; Zhu, Y. Visible light photoactivity enhancement via CuTCPP hybridized g-C3N4 nanocomposite. Appl. Catal. BEnviron. 2015, 166, 366-373. (84) Gu, Q.; Liao, Y.; Yin, L.; Long, J.; Wang, X.; Xue, C. Template-free synthesis of porous graphitic carbon nitride microspheres for enhanced photocatalytic hydrogen generation with high stability. Appl. Catal. B-Environ. 2015, 165, 503-510. (85) Yu, J.; Jin, J.; Cheng, B.; Jaroniec, M. A noble metal-free reduced graphene oxideCdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel. J. Mater. Chem. A 2014, 2, 3407-3416. (86) Xiao, X.; Wei, J.; Yang, Y.; Xiong, R.; Pan, C.; Shi, J. Photoreactivity and Mechanism of g-C3N4 and Ag Co-Modified Bi2WO6 Microsphere under Visible Light Irradiation. Acs Sustain. Chem. Eng. 2016, 4, 3017-3023. (87) Bai, X.; Wang, L.; Wang, Y.; Yao, W.; Zhu, Y. Enhanced oxidation ability of g-


38

Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C3N4 photocatalyst via C-60 modification. Appl. Catal. B-Environ. 2014, 152, 262-270. (88) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-free efficient photocatalyst for stable visible water splitting via a twoelectron pathway. Science 2015, 347, 970-974. (89) Wan, W.; Yu, S.; Dong, F.; Zhang, Q.; Zhou, Y. Efficient C3N4/graphene oxide macroscopic aerogel visible-light photocatalyst. J. Mater. Chem. A 2016, 4, 7823-7829.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

For Table of Contents Use Only

Fabricating

the

robust

g-C3N4

nanosheets/carbons/NiS

multiple

heterojunctions for enhanced photocatalytic H2 generation: An Insight into the tri-functional roles of nanocarbons

Jiuqing Wen,a,b,cJun Xie,a,b,c Zhuohong Yang,a, c Rongchen Shen,a,b,c Huiyi Li,a,b,c XingYi Luo,a, c Xiaobo Chen,d Xin Lia,b,c *

The tri-functional roles of nano-carbons in enhancing photocatalytic H2-generation activity over multiheterostructured g-C3N4/C/NiS nanocomposites were highlighted.


40

NiS Multiple

Recommend Documents