Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen

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Energy, Environmental, and Catalysis Applications

Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production on CdS/Cu7S4/g-C3N4 Ternary Heterostructures Jiayu Chu, Xijiang Han, Zhen Yu, Yunchen Du, Bo Song, and Ping Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02984 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production on CdS/Cu7S4/g-C3N4 Ternary Heterostructures Jiayu Chu,† Xijiang Han,*,† Zhen Yu,† Yunchen Du, † Bo Song*,‡ and Ping Xu*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡

Academy of Fundamental and Interdisciplinary Sciences, Department of Physics, Harbin

Institute of Technology, Harbin 150001, China

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (P.X.); [email protected] (X.H.); [email protected] (B.S.)

KEYWORDS: CdS/Cu7S4/g-C3N4, noble-metal-free, photocatalytic hydrogen production, visible light, ternary heterostructures

ACS Paragon Plus Environment

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ABSTRACT: Hydrogen production through photocatalytic water splitting has attracted much attention because of its potential to solve the issues of environmental pollution and energy shortage. In this work, CdS/Cu7S4/g-C3N4 ternary heterostructures are fabricated by ion exchange between CdS and Cu+, and subsequent ultrasonication-assisted self-assembly of CdS/Cu7S4 and g-C3N4, which provide excellent visible-light photocatalytic activity for hydrogen evolution without any noble metal co-catalyst. With the presence of p-n junction, tuned bandgap alignments and higher charge carrier density in the CdS/Cu7S4/g-C3N4 ternary heterostructures that can effectively promote the spatial separation and prolong the lifetime of photogenerated electrons, a high hydrogen evolution rate of 3570 µmol g-1 h-1, an apparent quantum yield (AQY) of 4.4% at 420 nm and remarkable recycling stability are achieved. We believe the as-synthesized CdS/Cu7S4/g-C3N4 ternary heterostructures can be promising noblemetal-free catalysts for enhanced hydrogen production from photocatalytic water splitting.


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1. Introduction Due to the increasing energy crisis and environmental pollution from fossil fuels, hydrogen energy has been considered as one of the most attractive clean energy sources because of its high energy capacity and environmental benignity.1-2 Production of hydrogen from photocatalytic water splitting is regarded as a promising strategy to respond to the energy crisis.3 Since the realization of water splitting on TiO2,4 various potential semiconductor catalysts have been investigated to further improve the photocatalytic efficiency of hydrogen production.5-8 Among them, CdS, due to its proper bandgap of 2.4 eV and excellent transport properties, is an attractive photocatalyst for converting solar energy into chemical fuels under visible light irradiation.7, 9-10 However, several issues have limited the hydrogen production rate on bare CdS materials, e.g. the low activity due to the rapid electron-hole recombination,

11-12

and serious inherent photo-

corrosion under strong illumination.13-14 Consequently, great efforts have been devoted to improving the separation of charge carriers and thus photocatalytic performance of CdS-based materials. One possible path to boost the photocatalytic activity is to construct CdS heterojunctions with other co-catalysts, enabling prolonged lifetime of the photogenerated electrons and holes.15-20 Generally, most of the co-catalysts are often composed of noble metal nanostructures (Pt, Pd, Ru, and Ag), but their scarcity and high price seriously hinder their practical applications.21-22 Recently, copper sulfide has been reported as a feasible co-catalyst because of its advantages of narrow bandgap, low-cost and earth abundance.23-26 Yu et al. reported that nonstoichiometric copper sulfide nanocrystals showed plasmonic absorption in the near infrared (NIR) region, which is attractive for solar-driven applications.27 In addition, copper sulfide nanocatalysts coupled with other materials (such as noble metal,28-29 ZnS,30-31 CdS,32-33 and Cu2O34) have also


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been reported in recent years, and these heterojunctions have displayed markedly enhanced photocatalytic efficiency. Therefore, preparation of CdS/copper sulfide nanocomposites with wide solar energy absorption range would be more attractive owing to the enhanced catalytic performance through the p-n heterojunction. However, the complicated synthesis procedure and relatively low hydrogen production rate have limited the application of CdS-based copper sulfide materials.24, 35 Meanwhile, the unsatisfying stability of the sulfide materials remains a common problem to be conquered.24-25, 36 Graphitic carbon nitride (g-C3N4) has been a highly studied material in the field of photocatalysis due to its unique characteristics: abundance, thermal and chemical stability, and visible-light response.7, 37-38 Some researchers have introduced it into the photocatalytic system to substantially reinforce the stability and activity of the photocatalysts. For instance, Zhang et al. have successfully developed a series of amorphous silver silicates/ultrathin g-C3N4 nanosheet (a-AgSiO/CNNS) heterojunction composites through an in situ precipitation method, which showed superior visible-light photocatalytic activities due to the synergetic effect between aAgSiO and CNNS.39 Ye et al. presented a modified g-C3N4 with porous structure, intrinsic electronic/band structure modulation, and up-shifted conduction band with strong reducing ability.40 Chen et al. successfully prepared Ag nanoparticle modified mesoporous C3N4 by photoassisted reduction, and a significant promotion for photoelectric conversion were achieved as a consequence of enhanced separation of the photogenerated electron-hole pairs and improved lifetime of the photogenerated electrons.41 To solve the low efficiency and poor stability issues of sulfide photocatalysts, introduction of a third component, g-C3N4, might be a good choice, as the band-edge offset between g-C3N4 and sulfide materials can effectively accelerate the


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separation of photogenerated charge carriers and alleviate the damage to sulfide materials by photogenerated holes.42 In this work, we demonstrate the fabrication of CdS/Cu7S4/g-C3N4 ternary heterostructures by a simple partial ion-exchange method and subsequent self-assembly procedure. From an ionexchange process between CdS nanorods and Cu+ ions, unique CdS/Cu7S4 nanorod structures with tuned composition and wide solar absorption range can be achieved. Subsequently, g-C3N4 is introduced through an ultrasonication-assisted self-assembly procedure, leading to the formation of CdS/Cu7S4/g-C3N4 ternary heterostructures consisting of CdS/Cu7S4 nanorods wrapped by g-C3N4 nanosheets. Notably, without the need of any noble metal (H2PtCl6) as cocatalyst, the CdS/Cu7S4/g-C3N4 ternary heterostructures exhibit excellent visible-light photocatalytic hydrogen evolution activity. We believe that this work can provide new insight for the design and synthesis of noble-metal-free photocatalysts for sustainable H2 production. 2. Experimental Section Synthesis of CdS Nanorods. CdS nanorods were fabricated through a hydrothermal method.43 Briefly, 1.87 g Cd(NO3)2·4H2O and 1.38 g NH2CSNH2 were ultrasonically dispersed into 30 mL ethylenediamine, and then the solution was transferred into a Teflon-liner and maintained at 160°C for 24 h. After that, the obtained yellow precipitates were centrifuged, rinsed with distilled water and ethanol for several times, and then dried in a vacuum drier at 60 °C. Synthesis of CdS/Cu7S4 Nanocomposites. CdS/Cu7S4 nanocomposites were synthesized at room temperature through a partial ion exchange process between CdS and Cu+ ions. Specially, a varying amount of [MeCN]4CuPF6 dissolved in 20 mL methanol was dropwise added into a solution of 300 mg CdS nanorods dispersed in 50 mL methanol to produce CdS/Cu7S4-x


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nanocomposites, where x is the molar ratio of Cu to Cd. After being stirred for 12 h, the CdS/Cu7S4 nanocomposites were collected and washed with methanol for several times before being vacuum dried. The compositions of the CdS/Cu7S4-x nanocomposites, prepared at different precursor molar ratios of Cu to Cd, were determined by X-ray photoelectron spectroscopy (XPS) and Inductively coupled plasma atomic emission spectroscopy (ICP- AES) techniques, and the results are summarized in Table S1. Here, the x values are shown according to the XPS results. Synthesis of CdS/Cu7S4/g-C3N4 ternary heterostructures. The pristine g-C3N4 was prepared by heating melamine at 530°C in nitrogen for 4 h at 5 °C min-1.40, 44 CdS/Cu7S4/g-C3N4 ternary heterostructures were fabricated as follows: 5 mg of as-prepared g-C3N4 was dispersed in 25 mL methanol. After being ultrasonically treated for 30 min, 25 mL methanol containing 100 mg CdS/Cu7S4 nanorods were dropwise added into the above solution, and then ultrasonically stirred at room temperature for 24 h. The obtained CdS/Cu7S4/g-C3N4 ternary heterostructures were collected and dried in a vacuum drier at 60°C for further experiments. Characterization. Scanning electron microscopy (SEM) images were obtained on a HELIOS NanoLab 600i (FEI). Transmission electron microscopy (TEM) images were obtained on a TECNAI F20 (FEI Instruments) at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) data was recorded on a Rigaku D/MAXRC X-ray diffractometer with Cu Kα radiation source (45.0 kV, 50.0 mA). XPS was performed on a PHI-5400 ESCA System using an Al Kα radiation as a source. ICP- AES was measured on a Perkin Elmer (PE) Optima 8300. The binding energy was calibrated using the C 1s photoelectron peak at 284.6 eV as the reference. The UVVis-NIR diffusion reflectance spectra were collected on a Hitachi UH4150 spectrometer. Photoluminescence (PL) spectra were recorded on a Perkin Elmer LS55 spectrometer in the wavelength range of 450-800 nm, using an excitation wavelength of 450 nm. Nitrogen


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adsorption/desorption

isotherms

were

collected

on

a

QUADRASORB

SI-KR/MP

(Quantachrome, USA) after heating the materials in vacuum for 2 h at 120 °C. Photocatalytic Hydrogen Production Test. The photocatalytic reactions were conducted on a vacuum-closed gas-circulation system with a top window at room temperature, which was sealed with silicone rubber septum. A 300W Xe lamp (PLS-SXE300/300UV) with a cut-off filter (>420 nm) was employed, which was positioned 10 cm away from the reactor, and the intensity focused on the reaction system was measured to be ~180 mW cm-2. In a typical procedure, 50 mg of the as-prepared catalyst were dispersed into 100 mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3. Prior to irradiation, the reaction system was pumped to vacuum for about 45 min. The amount of produced H2 was measured by gas chromatography (Techcomp GC7900) with a thermal conductivity detector (TCD), with N2 as the carrier gas. Here, no noble metal co-catalyst (H2PtCl6) was added into the reaction system. Apparent quantum yield (AQY) for hydrogen production at a fixed wavelength of 420 nm according to the following equation (1):

‫= ܻܳܣ‬

ே೐ ே೛

× 100% =

ଶெேಲ ௛௖ ௌ௉௧ఒ

× 100%

(1)

where Ne is the amount of reaction electrons, Np is the amount of incident photons, M is the amount of H2 molecules, NA is Avogadro’s constant, h is the Planck constant, c is the speed of light, S is the irradiation area, P is the intensity of the irradiation light, t is the reaction time, and λ is the wavelength of the incident light (420 nm). 3. Results and Discussion


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In order to construct a photocatalyst with wide solar absorption and rapid electron transportation, CdS/Cu7S4/g-C3N4 ternary heterostructures were designed and fabricated (Scheme 1). Visiblelight-active CdS nanonods were prepared through a hydrothermal route, and then Cu7S4 with NIR light absorption was grown on CdS nanorods by a simple ion exchange process, leading to the formation of CdS/Cu7S4 nanocomposites.45 This combination can not only extend the light absorption range but also create an interface for spatial charge separation. The CdS/Cu7S4/gC3N4 ternary heterostructures could be obtained through an ultrasonication-assisted selfassembly procedure, which is believed to improve the rate of hydrogen evolution and meliorate the photocorrosion issue of sulfide materials.

Scheme 1. Schematic illustration of preparing CdS/Cu7S4/g-C3N4 ternary heterostructures through an ion exchange process between CdS and Cu+ and subsequent ultrasonication-assisted self-assembly of CdS/Cu7S4 and g-C3N4.

The morphologies of CdS nanorods, CdS/Cu7S4 nanocomposites and CdS/Cu7S4/g-C3N4 ternary heterostructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a and Figure S1a, CdS nanorods with smooth surface have an average diameter of 20–50 nm and length up to 400-1000 nm. The ion exchange process between Cu+ ions and CdS leads to porous and roughened structures of the CdS/Cu7S4 nanocomposites, and the length of the CdS/Cu7S4 structure is getting shorter with the increase in the Cu content (Figure S1b-f). The successful formation of Cu7S4 on CdS nanorods


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can be reflected by the color change with the increase of Cu+ ion content (Figure S2). Take CdS/Cu7S4-0.33 as an example (Figure 1b), the surface has been greatly roughened compared to that of the pristine CdS nanorods due to the ion-exchange process. HR-TEM image in Figure 1c clearly shows the lattice fringes of 0.316 and 0.245 nm that can be assigned to the (101) plane of hexagonal CdS and (155) plane of monoclinic Cu7S4, respectively, confirming the formation of CdS/Cu7S4 nanocomposites. The as-prepared CdS/Cu7S4-0.33 nanocomposite was further analyzed by high-angle annular dark field scanning transmission electron microscopy (HAADFSTEM, Figure 1e), and energy-dispersive spectroscopy (EDS) mapping shows that Cd, Cu and S elements are uniformly distributed throughout the nanorods (Figure 1 f-h). Here, the closely linked heterojunctions between CdS and Cu7S4 may facilitate the rapid transport of electrons.28, 46 Subsequently, the CdS/Cu7S4 nanorods are well wrapped by the g-C3N4 nanosheets in the CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure through the ultrasonication-assisted selfassembly procedure (Figure 1d and S3), which is conducive to the stability during photocatalytic process and can further increase the visible light absorption due to the unique characteristics of g-C3N4.7, 40, 44, 47


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Figure 1. TEM images of (a) CdS nanorods and (b) CdS/Cu7S4-0.33 nanocomposites; (c) HR-TEM image of the CdS/Cu7S4-0.33 nanocomposites; (d) TEM image of CdS/Cu7S4/g-C3N4 ternary heterostructures; (e) HAADF-STEM image and (f-h) EDS mapping of a single CdS/Cu7S4-0.33 ternary heterostructure.

The crystal structures of the as-prepared CdS, g-C3N4, CdS/Cu7S4 and CdS/Cu7S4/g-C3N4 ternary heterostructures were examined by powder X-ray diffraction (XRD) (Figure 2). The diffraction peaks of the CdS nanorods can be well indexed to the hexagonal CdS phase (JCPDS No. 41-1049). From the XRD patterns of CdS/Cu7S4 nanocomposites, one can find that no characteristic peaks attributed to Cu7S4 can be detected from CdS/Cu7S4-0.09 to CdS/Cu7S4-0.45, probably due to the low content of Cu7S4 and strong diffraction of the CdS nanorods. When the molar ratio of Cd to Cu increases to 1:0.57, typical diffraction peaks at 2θ= ~46.8° and 48.9° corresponding to the (086) and (886) planes of monoclinic Cu7S4 (JCPDS No. 23-0958) are observed (inset in Figure 2).29 Two diffraction peaks at 13.2° and 27.4° are ascribed to the (100) and (002) peaks of the pure g-C3N4. However, no notable diffraction peaks of g-C3N4 can be found in the XRD pattern of the CdS/Cu7S4-0.33/g-C3N4 material, possibly resulting from the low content and diffraction intensity of g-C3N4.48


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Figure 2. XRD patterns of CdS, g-C3N4, CdS/Cu7S4 and CdS/Cu7S4/g-C3N4 nanocomposites.

In order to confirm the presence of g-C3N4 and Cu7S4, XPS measurement of CdS/Cu7S40.33/g-C3N4 ternary heterostructure was employed (Figure 3), and the survey spectrum in Figure 3a clearly shows the existence of Cu, Cd, C, N and S elements. In detail, the peaks at 405.1 and 411.8 eV in Cd 3d XPS spectrum are attributed to Cd 3d5/2 and Cd 3d3/2 of Cd2+ ions in CdS, respectively (Figure 3b).5, 14 The binding energies recorded at 931.8 and 951.9 eV in Cu 2p XPS spectrum and 161.4 and 162.5 eV in S 2p XPS spectrum are consistent with Cu7S4 (Figure 3c, d).29, 49 The characteristic peaks of C 1s XPS spectrum located at 284.7, 286.1 and 288.6 eV agree well with the sp2 C-C bonds, sp3 coordinated carbon species from the defects of g-C3N4, and the sp2-bonded carbon (N-C=N) in g-C3N4, respectively (Figure 3e, f).50-51 The three peaks at 398.5, 399.8 and 400.5 eV in N 1s XPS spectrum belong to N atoms bonded to two carbon atoms in sp2 configuration (C-N=C), the bridging tertiary N (N-(C)3), and amino functional groups having a hydrogen atom (C-N-H), respectively.52 The above results again indicate the successful fabrication of CdS/Cu7S4/g-C3N4 ternary heterostructures.


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Figure 3. (a) Survey XPS spectrum of CdS/Cu7S4-0.33/g-C3N4 ternary heterostructures , and high resolution XPS spectra of (b) Cd 3d, (c) Cu 2p, (d) S 2P, (e) C 1s, and (f) N 1s.

Optical absorption features of the photocatalysts are crucial to their photocatalytic properties. Figure 4 shows the comparison of the UV-Vis-NIR diffuse reflectance spectra of CdS, CdS/Cu7S4-0.33 and CdS/Cu7S4-0.33/g-C3N4 heterostructure. The absorption edge for pure CdS at 520 nm corresponds to its bandgap energy of 2.4 eV, and CdS/Cu7S4-0.33 nanocomposites show absorption from the visible light region to the NIR light region due to the plasmonic absorption of Cu7S4,27,

53-54

which is beneficial for enhancing the catalytic activity of

photocatalysts. Furthermore, an enhanced absorbance in the NIR region is revealed with increasing Cu7S4 content in the CdS/Cu7S4–x nanocomposites (x=0, 0.09, 0.18, 0.33, 0.45, 0.57), which indicates that the incorporation of Cu7S4 can extend the absorption region effectively (Figure S4). Meanwhile, the introduction of g-C3N4 further enhances the optical absorption of the CdS/Cu7S4-0.33/g-C3N4 heterostructure in the visible and NIR region, which may possibly enhance the photocatalytic activity.


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Figure 4. Comparison of UV-Vis-NIR diffuse reflectance spectra of the prepared CdS, CdS/Cu7S4-0.33 and CdS/Cu7S4-0.33/g-C3N4 heterostructure.

The specific surface

areas

of

CdS, CdS/Cu7S4-0.33

and

CdS/Cu7S4-0.33/g-C3N4

heterostructure were determined by the nitrogen adsorption-desorption isotherms based on the Brunauer-Emmett-Teller (BET) model (Figure S5). Obviously, these nanocomposites show type IV adsorption-desorption isotherms according to the IUPAC classification.55-56 CdS nanorods prepared by hydrothermal method have relatively smooth surface with a BET surface area of 18 m2 g-1. The CdS/Cu7S4-0.33 nanocomposites have a larger surface of 30 m2 g-1 due to the coarse surface formed on the CdS/Cu7S4-0.33 nanocomposites during the ion exchange process, and the CdS/Cu7S4-0.33/g-C3N4 has the largest surface of 37 m2 g-1 due to the addition of g-C3N4. A higher specific surface area of the CdS/Cu7S4-0.33/g-C3N4 ternary heterostructures can provide more active sites and facilitate the charge carrier transport process. To verify the superiority of using CdS/Cu7S4/g-C3N4 ternary heterostructures as photocatalysts, a series of photocatalytic hydrogen evolution experiments were conducted under


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visible light (>420 nm) irradiation (Figure 5). It should be pointed out that in our experiments, no conventional noble metal co-catalyst (H2PtCl6) is adopted. By adjusting the content of Cu7S4, the hydrogen evolution on CdS/Cu7S4 nanocomposites can be effectively tuned (Figure 5a), and the hydrogen evolution rates of different photocatalysts were compared in Figure 5b. Pure CdS sample exhibits a photocatalytic H2 production activity of 162 µmol g-1 h-1. The H2 evolution rate is markedly enhanced with the introduction of Cu7S4 as a co-catalyst, reaching a value as high as 1930 µmol g-1 h-1 with a molar ratio of Cd to Cu at 1:0.33. Obviously, this H2 evolution efficiency is much better than that of pure CdS, g-C3N4 (14.8 µmol g-1 h-1), CdS/g-C3N4 (920 µmol g-1 h-1) (Figure S6) and even CdS/g-C3N4/Pt (1710 µmol g-1 h-1), implying that Cu7S4 works much better than noble metal Pt as a co-catalyst in this system. However, further increasing the content of Cu7S4 results in a reduced photocatalytic activity. In addition, we prepared pure Cu7S4 successfully by adding excessive Cu+ ions during the ion exchange process with CdS (Figure S7 and Figure S8). However, there is no hydrogen evolution even after five hours under our experimental conditions by using pure Cu7S4 as the only catalyst. The rapid recombination of photogenerated electrons and holes may be caused by the narrow bandgap of Cu7S4 (~0.48 eV) (Figure S9). Importantly, hydrogen production rates of CdS/Cu7S4 nanocomposites are greatly enhanced after coupling with the g-C3N4 nanosheets, and the maximum can reach 3570 µmol g-1 h-1 on CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure. Notably, this high H2 evolution rate from CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure surpasses that of most previous results (Table S2).24,

32, 35

In the CdS/Cu7S4/g-C3N4 ternary heterostructures, CdS, Cu7S4 and g-C3N4 have

excellent absorption ability under Vis-NIR light illumination, and the formed heterojunction among them also promote the transfer of electrons and holes.


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Figure 5. (a) Comparison of photocatalytic H2 generation activities of CdS/Cu7S4 with different molar ratios of Cd to Cu (with and without g-C3N4). (b) Photocatalytic hydrogen evolution activities of different catalysts under visible-light irradiation (> 420 nm). (c) Photocurrent responses for CdS (black), CdS/Cu7S4-0.33 (red) and CdS/Cu7S4-0.33/g-C3N4 heterostructure (blue). (d) Recycled hydrogen evolution property of CdS/Cu7S40.33 (red) and CdS/Cu7S4-0.33/g-C3N4 heterostructure (blue) for 15 h.

The behavior of photogenerated electron transfer in the CdS, CdS/Cu7S4 and CdS/Cu7S4/gC3N4 photocatalysts was verified by the photocurrent responses under visible light irradiation. As shown in Figure 5c, CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure exhibits higher photocurrent density than CdS and CdS/Cu7S4-0.33, an indication that the construction of the heterojunctions can effectively strengthen the electron–hole separation and further improve the photocatalytic activity greatly. Moreover, the photoluminescence (PL) emission spectra were applied to explore the charge trapping and separation in CdS, CdS/Cu7S4-0.33 and CdS/Cu7S4-0.33/g-C3N4 photocatalysts (Figure S10). It is found that the introduction of Cu7S4 and g-C3N4 have notable effects on the PL peak intensity of CdS, and the PL intensity of CdS/Cu7S4-0.33/g-C3N4 ternary


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heterostructure is lower than that of CdS and CdS/Cu7S4-0.33, which restrains the recombination of electron–hole pairs and consequentially increases the lifetime of the photogenerated electrons. To further probe the charge-transfer process, the density of charge carrier (Nd) of the photocatalysts are calculated and compared by Mott-Schottky analysis (Figure 6). Carrier density can be calculated from Equation (2)57

Nd =

2 e0εε 0 d (1 C 2 ) dV

(2)

where e0 is the electron charge, ε is the dielectric constant of semiconductor, ε0 is the permittivity of vacuum, C is the capacitance, V is the applied bias at the electrode. Here we take ε=11.6 for CdS. The CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure shows a substantially smallest slope than that of CdS/Cu7S4-0.33 and CdS, suggesting an increase in carrier density. The carrier density of CdS/Cu7S4-0.33/g-C3N4, CdS/Cu7S4-0.33 and CdS is calculated to be approximately 1.03×1019, 8.22×1017 and 5.14×1017 cm-3, respectively. The major reason of the high carrier density of CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure is that the presence of Cu7S4 and gC3N4 in CdS/Cu7S4/g-C3N4 ternary heterostructures can improve the charge transport and dramatically suppress the electron–hole recombination. Moreover, the bandgap of the CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure is reduced, which may substantially enhance the photocatalytic activity.


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Figure 6. Mott-Schottky plots of CdS, CdS/Cu7S4-0.33 and CdS/Cu7S4-0.33/g-C3N4.

Meanwhile, stability of the photocatalysts for prolonged irradiation of 15 h was also studied (Figure 5d). There is no significant activity loss of CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure after five cycles. In comparison, decreased hydrogen production rate can be seen from the third cycle of CdS/Cu7S4-0.33 nanocomposites, indicating that g-C3N4 can effectively accelerate the separation of photogenerated charge carriers and alleviate the damage to sulfide materials. In addition, there is no obvious change in the morphology of the CdS/Cu7S4-0.33/gC3N4 ternary heterostructure after five cycles (Figure S11). Also, there are no obvious changes in the XRD patterns (Figure S12) as well as the binding energies of Cd, Cu, S, C and N elements in the XPS spectra (Figure S13) of the used CdS/Cu7S4-0.33/g-C3N4 ternary heterostructure, revealing the high stability of this ternary heterostructure during the photocatalytic hydrogen evolution process.58-60 Based on above results, the photocatalytic mechanism is proposed. By combining the bandgap energies and estimated the conduction band potentials of the three components in the heterostructure, the corresponding band structures are established in Figure 7. The optical


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bandgaps for CdS, Cu7S4 and g-C3N4 are ~2.41 eV, ~0.48 eV and ~2.85 eV, respectively, and the Mott-Schottky plots indicate that the estimated conduction band potential of Cu7S4 lies below that of CdS and g-C3N4 (Figure S9). Therefore, the photogenerated electrons from the conduction band (CB) of g-C3N4 can be directly transferred to the CB of CdS, and then to the CB of Cu7S4 under visible light irradiation.27, 29, 61 It is noteworthy that the slope of Mott-Schottky curves for CdS and g-C3N4 are positive and Cu7S4 is negative, confirming that CdS and g-C3N4 are n-type conductor, and Cu7S4 is p-type conductor.17 In addition, the separation of electron-hole pairs can be accelerated due to the formed p–n junction between CdS and Cu7S4, and once g-C3N4 is introduced to the system, it can bring more heterojunction spots and utilize more solar radiation, which effectively increases the charge carrier lifetime to protect CdS and Cu7S4 from possible photocorrosion. Therefore, enhanced hydrogen evolution and good photochemical stability are achieved on the ternary nanocomposite photocatalysts.

Figure 7. Schematic illustration of the possible visible-light photocatalytic charge transfer and hydrogen evolution processes in the CdS/Cu7S4/g-C3N4 ternary heterostructures.


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Conclusions In conclusion, CdS/Cu7S4/g-C3N4 ternary heterostructures with enhanced Vis-NIR light absorption are fabricated successfully. With tuned bandgap alignment and formed p–n junction that can effectively separate the photogenerated electrons and holes and prolong the lifetime of the photogenerated electrons, photocatalytic hydrogen production activity of CdS/Cu7S4/g-C3N4 ternary heterostructures is dramatically enhanced, and the highest rate of hydrogen evolution is 3570 µmol g-1 h-1 without the presence of conventional noble-metal co-catalyst, which is superior to most of the reported CdS-based photocatalysts. Designed synthesis of ternary heterostructures with broadened light absorption may pave new avenues for further boosting the photocatalytic researches.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details and additional characterizations (PDF) AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank the financial support from the National Natural Science Foundation of China (21471039, 21571043, 21671047), Fundamental Research Funds for the Central Universities (PIRS of HIT A201502 and HIT. BRETIII. 201223), China Postdoctoral Science Foundation


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(60) Vamvasakis, I.; Trapali, A.; Miao, J.; Liu, B.; Armatas, G. S. Enhanced Visible-Light Photocatalytic Hydrogen Production Activity of Three-Dimensional Mesoporous p-CuS/n-CdS Nanocrystal Assemblies. Inorg. Chem. Front. 2017, 4, 433-441. (61) Shang, L.; Tong, B.; Yu, H.; Waterhouse, G. I. N.; Zhou, C.; Zhao, Y.; Tahir, M.; Wu, L.Z.; Tung, C.-H.; Zhang, T. CdS Nanoparticle-Decorated Cd Nanosheets for Efficient Visible Light-Driven Photocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1501241.


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Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen

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