Enhanced Visible-Light-Driven Photocatalytic H2 Evolution from Water

Apr 12, 2017 - Fan Zhang , Hua-Qiang Zhuang , Jie Song , Yu-Long Men , Yun-Xiang Pan , Shu-Hong Yu. Applied Catalysis B: Environmental 2018 226, 103- ...
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Research Article pubs.acs.org/journal/ascecg

Enhanced Visible-Light-Driven Photocatalytic H2 Evolution from Water on Noble-Metal-Free CdS-Nanoparticle-Dispersed Mo2C@C Nanospheres Yun-Xiang Pan,*,†,‡ Jun-Bao Peng,† Sen Xin,§ Ya You,*,§ Yu-Long Men,† Fan Zhang,† Ming-Yu Duan,† Yu Cui,† Zheng-Qing Sun,† and Jie Song*,∥ †

School of Chemistry and Chemical Engineering, Hefei University of Technology, No. 193, Tunxi Road, Hefei 230009, P. R. China State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, No. 79, West Yingze Street, Taiyuan 030024, P. R. China § Department of Mechanical Engineering, The University of Texas at Austin, 1 University Station C2200, Austin, Texas 78712-0292, United States ∥ Institute of Nano Biomedicine and Engineering, Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, No. 800, Dongchuan Road, Shanghai 200240, P. R. China ‡

S Supporting Information *

ABSTRACT: Developing efficient noble-metal-free catalysts for photocatalysis under the irradiation of visible light, which is the main part of sunlight (44%), would represent a significant step toward making photocatalysis a more competitive strategy for solar energy utilization. Herein, nanospheres (∼200 nm) containing dimolybdenum carbide and carbon (Mo2C@C) were used to support CdS nanoparticles (∼5 nm) to form a noble-metal-free CdS/Mo2C@C photocatalyst. CdS/Mo2C@C shows an enhanced visible-light-driven photocatalytic H2 evolution from water, with a H2 evolution rate of 554.3 μmol h−1, which is about 2 times higher than that on the widely used noble-metal-based CdS/Pt photocatalyst. Improved absorption of the visible light and separation of the photogenerated electron−hole pairs could be the origins for the enhanced photocatalytic activity of CdS/Mo2C@C. The findings of this work will open a new door for fabricating efficient noble-metal-free photocatalysts for visible-light-driven photocatalysis. KEYWORDS: Visible-light-driven photocatalysis, Molybdenum carbide, Charge separation, H2, Water splitting

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low reserves and high prices.8−14 Low-cost transition metal carbides, such as dimolybdenum carbide (Mo2C), are attractive alternatives to noble metals for photocatalysis.26−32 Modification of the electronic properties of the transition metal by the carbon atom via charge transfer and structural change makes the carbides similar to noble metals.32,33 In addition, the carbides are conductive, benefiting for the charge separation in photocatalysis.32,33 Yet, to date, the photocatalytic efficiency of the carbides is still too low and requires further improvement. Herein, nanospheres (∼200 nm) consisting of Mo2C and carbon, denoted by Mo2C@C, were used to support cadmium sulfide (CdS) nanoparticles (∼5 nm) to form a noble-metalfree photocatalyst, named as CdS/Mo2C@C. In visible-lightdriven photocatalytic H2 evolution from water, CdS/Mo2C@C shows a H2 evolution rate of 554.3 μmol h−1, which is about two times higher than that on the noble-metal-based photo-

INTRODUCTION Photocatalysis driven by solar energy is promising for solving the energy and environmental crises.1−21 However, there is still a long way to go for applications of photocatalysis on massive scales. A challenge for the photocatalysis is that the photocatalytic efficiency is far below the requirement of massive-scale applications.4,5 The poor ability of the photocatalyst in absorbing visible light, which is the main part of the sunlight (44%), is one of the origins for the low photocatalytic efficiency.4,5 The ability of the photocatalyst in separating the photogenerated electron−hole pairs is also a key factor affecting the photocatalytic efficiency.4−8 A poor charge separation decreases the amount of the charge carriers required for photocatalysis and, thus, lowers the photocatalytic efficiency. Photocatalyst with high abilities in visible light absorption and charge separation is thereby highly desired. Another challenge for the photocatalysis is the use of noble metals, such as Pt, as the cocatalysts or active sites.4,5,22−25 The noble metals can indeed promote the photocatalytic reactions but are unsuitable for massive-scale applications, due to their © 2017 American Chemical Society

Received: March 14, 2017 Revised: April 5, 2017 Published: April 12, 2017 5449

DOI: 10.1021/acssuschemeng.7b00787 ACS Sustainable Chem. Eng. 2017, 5, 5449−5456

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Inductively coupled plasma mass spectrometry (ICP-MS) measurements indicate that the actual weight percentage of Pt on CdS/Pt is 0.5%. Characterizations. The scanning electron microscopy (SEM) was performed on a Sirion200 field emission scanning electron microscope. The transmission electron microscope (TEM), including highresolution TEM (HRTEM) and elemental mapping, were obtained on a JSM-6700F transmission electron microscope with a beam energy of 200 kV. X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO MPD diffractometer using Cu Kα radiation (I = 1.5406 Å, 40 kV and 20 mA). Raman spectra were collected on a LabRam HR Evolution with an argon laser source at 532 nm manufactured by HORIBA JOBIN YVON. UV-3101 Shimadzu spectrophotometer was applied to observe the UV−visible diffuse reflectance spectra of the photocatalysts. X-ray photoelectron spectroscopy (XPS) was done on an ESCALAB250Xi X-ray photoelectron spectrometer with a monochromatic X-ray source manufactured by Thermo. A Quantachrome Autosorb-6B apparatus was used to explore the nitrogen adsorption−desorption isotherm, Brunauer−Emmett−Teller (BET) surface area and pore size of the samples. Photoluminescence (PL) spectra were measured using a Jobin Yvon Fluorolog 3-TAU luminescence spectrometer (Jobin Yvon Instruments Co., Ltd., France). The ICP-MS measurements were conducted on a Thermo X series 2 ICP-MS spectrometer. Zeta potentials were measured by using a ZetaPALS Zeta Potential Analyzer (Brookhaven Instruments Corporation). Transient photocurrents were measured on a CHI660A electrochemical workstation in a Na2SO4 solution (0.1 M) containing lactic acid (10 vol %) using a 300 W Xe-lamp equipped with a cutoff filter (λ > 420 nm), with a constant potential of 0.35 V vs standard electrode (SCE). The solution and light source used for measuring the photocurrent are similar to those for the photocatalytic reactions. The electrochemical workstation includes three electrodes. Pt and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. For preparing the working electrode, with a surface area of 1.44 cm2 exposed to the electrolyte, 1 mg sample was first dispersed in 1 mL ethanol containing naphthol (0.05 wt %) under ultrasonication. Then, the suspension was dropped onto indium tin oxide (ITO) glasses and dried at room temperature. Visible-Light-Driven Photocatalytic Water Splitting. The lamp used in the photocatalytic reactions was a 300 W Xe-lamp with a cutoff filter (λ > 420 nm). A top window Pyrex cell (300 mL) connected to a closed gas circulation evacuation system was used as the photoreactor. Photocatalytic reactions were performed with a constant lamp power and a fixed distance between the reactor and the lamp (about 10 cm). In the photoreactor, the photocatalyst (100 mg) was dispersed in a water solution (100 mL) containing 10 vol % lactic acid as a hole scavenger. Before light irradiation, the reaction system was evacuated and refilled with argon several times to remove the air inside. The photocatalytic reactions were carried out at about 20 °C by using cooling water. The H2 evolved was detected by an online gas chromatography (Agilent 7890A, Thermal Conductivity Detector). The quantum efficiency (QE) was measured under the conditions same as the photocatalytic reactions with band-pass with center wavelength at 420, 440, 460, 480, 500, and 520 nm, respectively. The number of incident photons was measured with a silicon photodiode (13 DAS 005, MELLES GRIOT) connected to a broad-band power/ energy meter (13 PEM001, MELLES GRIOT). The QE was calculated by using eq 1:

catalyst formed by combining CdS with Pt nanoparticles (CdS/ Pt). Improved separation of the photogenerated electron−hole pairs and absorption of the visible light could be the origins for the enhanced photocatalytic performance of CdS/Mo2C@C.

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MATERIALS AND METHODS

Sample Preparation. The reagents used herein, including ammonium molybdate tetrahydrate (AMT, N6H24Mo7O24·H2O), sulfur, chloroplatinic acid hexahydrate (H2PtCl6·6H2O), glucose, CdO, oleic acid (OA), octadecene (ODE), trioctylphosphine (TOP), and mercaptopropionic acid (MPA), in analytical grade purity, were purchased from Alfa Aesar and used as received without further treatment. For synthesizing carbon nanospheres, glucose (1000 mg) was first dissolved in 45 mL deionized (DI) water at room temperature. And then, the solution was transferred into a 50 mL Teflon-lined stainless autoclave and kept at 180 °C for 10 h, followed by cooling to room temperature naturally. The precipitate from the hydrothermal process was washed with ethanol and DI water several times. After washing, the precipitate was dried under vacuum at 25 °C for 12 h and subsequently calcined at 800 °C for 2 h under argon atmosphere, producing the carbon nanospheres. For clarity, the carbon nanospheres were denoted by CNS. For preparing the Mo2C@C nanospheres, we first conducted a hydrothermal process, in which 45 mL of water solution containing AMT (23 mg) and glucose (1000 mg) was put into a 50 mL Teflonlined stainless autoclave and kept at 180 °C for 10 h. The as-prepared solid sample was then washed with ethanol and DI water for several times, followed by drying under vacuum at 25 °C for 12 h. The dried sample was calcined under argon atmosphere (800 °C, 2 h), leading to the formation of the Mo2C@C nanospheres. For comparison, we also prepared some other Mo2C-based materials by using the procedure similar to that of the Mo2C@C nanospheres, with different amounts of glucose (50, 100, 200, 300, 500, 1200, 1500, and 2000 mg) and the AMT amount fixed at 23 mg. To distinguish from the Mo2C@C nanospheres, the materials fabricated by using 50, 100, 200, 300, 500, 1200, 1500, and 2000 mg of glucose were denoted by M50, M100, M200, M300, M500, M1200, M1500, and M2000, respectively, where M is the abbreviation of the word material and the number after M corresponds to the glucose amount used. The CdS nanoparticles were produced via a hot-injection method which has been widely used.29 The method can be described briefly as follows. CdO (0.512 g), OA (5.0 mL), and ODE (15.0 mL) were mixed in a 3-neck flask (50 mL) and heated under an argon atmosphere. When the temperature of the mixture reaches to 300 °C, the TOP solution of sulfur (2 M, 1 mL) was quickly injected into the mixture, and the temperature was kept at 260 °C for 5 min, followed by cooling to room temperature naturally. This produced the CdS nanoparticles coated by TOP molecules. TOP molecules on the CdS nanoparticles were next exchanged by MPA molecules in methanol under stirring, producing the water-soluble CdS nanoparticles. For fabricating CdS/Mo2C@C, Mo2C@C nanospheres (15 mg) were first dispersed in water solution of CdS nanoparticles (2 mL, 15 mg mL−1) under ultrasonication for 30 min. Then, the mixture was centrifuged to collect the solid sample which was washed with ethanol and DI water for several times and subsequently dried under vacuum at 25 °C for 12 h, resulting in CdS/Mo2C@C. By using the same method, CdS/CNS, CdS/M50, CdS/M100, CdS/M200, CdS/M300, CdS/M500, CdS/M1200, CdS/M1500, and CdS/M2000 were also fabricated. CdS/Pt was prepared using the following steps. First, 1 mL of water solution of H2PtCl6·6H2O, with a Pt content of 0.16 mg mL−1, was added into water solution of CdS nanoparticles (2 mL, 15 mg mL−1), under ultrasonication for 30 min, depositing Pt ions onto the CdS nanoparticles. Then, the sample was irradiated for 1 h by using a 300 W Xe-lamp, reducing Pt ions into Pt0. Finally, the solid sample was harvested by centrifugation, washed with ethanol and DI water several times, and dried under vacuum at 25 °C for 12 h, fabricating CdS/Pt.

QE =

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2 × (the number of H 2 molecules produced) × 100% the number of incident photons (1)

RESULTS AND DISCUSSION Characterizations on CNS. The CNS has a diameter of about 500 nm (Figures 1a and b). On the XRD pattern of CNS (Figure 2a), there is a wide peak at 25° characteristic of carbon. Raman spectrum of CNS shows a band at 1345 cm−1 (D band) 5450

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Mo2C (PDF no. 350787), respectively. This demonstrates the presence of both carbon and Mo2C on Mo2C@C. Raman spectrum of Mo2C@C (Figure 2b) exhibits the D and G bands due to carbon, similar to that of CNS. This further indicates the presence of carbon on Mo2C@C. ICP-MS measurements imply that there is about 5.1 wt % of Mo in Mo2C@C. The Mo2C@C nanosphere has a core with a size of about 20−30 nm and a shell (Figure 1d). For identifying the core and shell, we analyzed M50, M100, M200, M300, and M500 using XRD patterns (Figure S1 in the Supporting Information) and TEM observations (Figure S2 in the Supporting Information). M50 and M100 are both composites including Mo2C and MoO2 (Figure S1). On the XRD pattern of M200, only peaks assigned to Mo2C are observed. TEM observations suggest that M200 are nanoparticles, with a size of 20−30 nm. The XRD patterns of M300 and M500 exhibit the peaks of Mo2C and carbon, similar to that of the Mo2C@C nanospheres (Figure 2a). M300 and M500 are both in a morphology of nanosphere with a core of 20−30 nm and a shell (Figure S2). As such, the optimum glucose amount for producing pure Mo2C is 200 mg. MoO2 appears with the glucose amount smaller than 200 mg, whereas carbon is present with the glucose amount larger than 200 mg. These results indicate that the Mo2C@C nanosphere could be formed via the following steps. First, part of the glucose is used to form the Mo2C nanoparticles. Then, the rest of glucose is converted into carbon and coat on the Mo2C nanoparticles. Thus, the core and shell of the Mo2C@C nanospheres could be Mo2C and carbon, respectively. In addition, we treated the Mo2C@C nanospheres by aqua regia under stirring. TEM observations imply that the core of the Mo2C@C nanospheres disappears after the aqua-regiatreatment (Figure S3 in the Supporting Information). For the material before the aqua-regia-treatment, the XRD peaks attributed to Mo2C and carbon are both present (Figure 2a); however, on the XRD pattern of the aqua-regia-treated

Figure 1. (a) SEM and (b) TEM images of CNS. (c) SEM and (d) TEM images of Mo2C@C.

caused by the disordered sp2 hybridized carbon network, and a band at 1583 cm−1 (G band) assigned to the ordered sp2 hybridized carbon network (Figure 2b).34,35 This implies the presence of both graphitic and nongraphitic carbon on CNS. Characterizations on Mo2C@C. As shown in Figures 1c and 1d, the Mo2C@C nanospheres have a diameter of about 200 nm. On the XRD pattern of Mo2C@C (Figure 2a), besides the peak at 25° assigned to carbon, the peaks at 34.4°, 52.1°, 61.5°, 69.6°, 74.6°, and 75.5° are due to the (100), (102), (110), (101), (103), (112), and (201) surfaces of the hexagonal

Figure 2. (a) XRD patterns of CNS, precursor of Mo2C@C, Mo2C@C, and CdS/Mo2C@C. (b) Raman spectra of CNS and Mo2C@C. (c) XPS spectrum of Mo 3d on Mo2C@C. (d) XPS spectrum of C 1s on Mo2C@C. 5451

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about 5 nm (Figure S5 in the Supporting Information) and have been studied in detail previously.29,40,41 The XRD pattern of CdS/Mo2C@C is shown in Figure 2a. On the XRD pattern of CdS/Mo2C@C, besides the peak attributed to carbon and Mo2C, peaks caused by standard cubic CdS are present at 26.5° and 44.0° (PDF no. 80-0019). Figure 4 shows the TEM images

material, only the peak due to carbon can be observed (Figure S4 in the Supporting Information). Thus, the component of the Mo2C@C nanospheres, which is not observed in the TEM image, could be Mo2C. These results further indicate that the core and shell of the Mo2C@C nanosphere could be Mo2C and carbon, respectively. We next analyze the Mo2C@C nanospheres using XPS spectra. Figure 2c shows the Mo 3d XPS spectrum for Mo2C@ C. The XPS spectrum can be fitted into peaks at 235.9, 234.1, 232.9, 232.7, 231.8, 230.1, 229.3, and 228.7 eV, respectively. The peaks at 235.9 and 232.9 eV are due to the 3d3/2 and 3d5/2 of Mo6+ in MoO3, respectively.36,37 The peaks at 234.1 and 230.1 eV are assigned to the 3d3/2 and 3d5/2 of Mo4+ in MoO2, respectively.27,36−38 The molybdenum carbide can be contaminated with MoO2 and MoO3 when it is exposed to the air.37,38 It is thereby reasonable to observe the Mo6+ and Mo4+ on Mo2C@C. The peaks at 232.7 and 229.3 eV are attributed to the 3d3/2 and 3d5/2 of Mo3+ possibly resulted from oxycarbide, respectively.36 The peak at 228.7 is caused by the 3d5/2 of Mo2+ in Mo2C.36 The peak at 231.8 eV corresponds to the 3d5/2 of Mo0 possibly due to the Mo−Mo bonds.37 On the XPS spectrum of C 1s on Mo2C@C (Figure 2d), the peak at 283.8 eV is caused by the carbidic carbon, the peaks at 288.6 and 286.7 eV is from the C−O and C−C bonds, respectively, and the peak at 284.9 eV is due to the sp2-hybridized polycyclic aromatic carbon (CC).36 As reflected by the XPS results, MoO3 and MoO2 appear on Mo2C@C. However, there is no MoO2 or MoO3 phase in the XRD pattern of Mo2C@C. This indicates that the MoO3 and MoO2 on Mo2C@C may be amorphous. Through an adsorption−desorption measurement (Figure 3a), the BET surface area of Mo2C@C was calculated to be 268.4 m2 g−1.39 The pore size distribution of Mo2C@C ranges from 3.6 to 19.3 nm, with a maximum at 4.9 nm (Figure 3b). Characterizations on CdS/Mo2C@C. The CdS nanoparticles used for fabricating CdS/Mo2C@C have a size of

Figure 4. (a, b) TEM images of CdS/Mo2C@C. (c) HRTEM image of CdS/Mo2C@C. (d) Elemental mapping patterns of CdS/Mo2C@ C.

and elemental mapping patterns for CdS/Mo2C@C. The lattice fringes with distances of 0.20 and 0.35 nm, observed in the TEM image, are due to the Mo2C(101) and CdS(111) surfaces, respectively. Elemental mapping patterns (Figure 4d) evidence the presence of Mo, C, Cd, and S on CdS/Mo2C@C. The XPS spectrum of Mo 3d on CdS/Mo2C@C (Figure S6 in the Supporting Information) can be fitted into peaks attributed to Mo6+ in MoO3, Mo4+ in MoO2, Mo3+ in oxycarbide, Mo2+ in Mo2C, and Mo0. On the XPS spectrum of C 1s on CdS/Mo2C@C (Figure S7 in the Supporting Information), the peaks caused by carbidic carbon, C−O bond, C−C bond, and CC bond can be observed. These results are similar to those of the Mo2C@C nanospheres (Figures 2c and d). The BET surface area of CdS/Mo2C@C was measured to be 276.1 m2 g−1, which is larger than that of the pure CdS nanoparticle (31.6 m2 g−1) and Mo2C@C nanospheres (268.4 m2 g−1). As reflected by the UV−visible spectra (Figure S8 in the Supporting Information), CdS/Mo2C@C has an excellent ability in absorbing visible light, with an absorption edge at about 520 nm, which is close to that of the pure CdS nanoparticles. Zeta potential measurements indicate that the surfaces of the pure CdS nanoparticles are negatively charged (−36.2 mV), whereas the Mo2C@C nanospheres exhibit a positive Zeta potential of about +42.1 mV. The CdS nanoparticles used herein are coated with mercaptopropionic acid (MPA) molecule. The two ends of the MPA molecule chain are thiol and carboxylic groups, respectively. MPA binds to the CdS nanoparticles via the thiol group. In water solution, the carboxylic group of MPA is deprotonated, making the CdS

Figure 3. (a) Nitrogen adsorption−desorption isotherm of the Mo2C@C nanosphere. (b) Pore size distribution on the Mo2C@C nanosphere. 5452

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ACS Sustainable Chemistry & Engineering nanoparticles negatively charged. As such, the CdS nanoparticles can be deposited easily on the surface of the Mo2C@C nanospheres by electrostatic interactions.40 We next explored the ability of CdS/Mo2C@C in separating the photogenerated electron−hole pairs, which is a key factor affecting the photocatalytic efficiency. CdS has a high PL at about 500 nm (Figure 5a), which results from the facile

Figure 6. (a) H2 evolution rates on CdS/Mo2C@C and control samples. (b) Photocatalytic stability of CdS/Mo2C@C and CdS/Pt. Reaction conditions: 100 mL aqueous solution with 10 vol % lactic acid, 100 mg photocatalyst, 300 W Xe-lamp (λ > 420 nm). The abbreviation evac. is short for “evacuation”.

There is no H2 detected on Mo2C@C. The pure CdS and CdS/CNS are photocatalytically active but exhibit H2 evolution rates as low as 21.5 and 34.9 μmol h−1, respectively. By varying the mass ratio of CdS to Mo2C@C for preparing CdS/Mo2C@ C, glucose amount, and calcination temperature for preparing Mo2C@C, an optimum H2 evolution rate of 554.3 μmol h−1 was obtained on CdS/Mo2C@C (Figures S9−S11 in the Supporting Information). This is much higher than those on the pure CdS and CdS/CNS (Figure 6a). For comparison, the photocatalytic activity of the widely used noble-metal-based CdS/Pt photocatalyst was also explored. By changing the Pt content on CdS/Pt, an optimal H2 evolution rate of 263.8 μmol h−1 was obtained (Figure S12 in the Supporting Information). The H2 evolution rate on CdS/ Mo2C@C is about 2 times higher than that on CdS/Pt. Therefore, the noble-metal-free Mo2C is a good alternative to the noble metal, i.e. Pt, to be used as cocatalysts providing active sites for the photocatalytic reactions. The lower photocatalytic activity of CdS/Pt may be due to the smaller surface area of CdS/Pt (46.9 m2 g−1) than that of CdS/Mo2C@ C (276.1 m2 g−1). The stability of CdS/Mo2C@C in H2 evolution was examined in three consecutive runs with each run of 4 h, and there is nothing else to do but evacuate the photoreactor between two runs. As shown in Figure 6b, CdS/Mo2C@C has an enhanced stability in H2 evolution, as compared with CdS/ Pt. The decrease of the total amount of H2 produced after three runs on CdS/Mo2C@C is only 3.5%, from 2136.6 μmol in the first run to 2062.0 μmol in the third run, whereas the decrease on CdS/Pt is as high as 21.5%. The QE of CdS/Mo2C@C has an optimum value of 33.4% at 460 nm (Figure S13 in the Supporting Information).

Figure 5. (a) PL spectra of the pure CdS nanoparticles and CdS/ Mo2C@C. (b) Photocurrent−time profiles of the pure CdS nanoparticles, CdS/M200, and CdS/Mo2C@C.

recombination of the photogenerated electron−hole pairs on CdS.40,41 As compared with CdS, the PL on CdS/Mo2C@C is greatly suppressed (Figure 5a), indicating a more efficient separation of the photogenerated electron−hole pairs on CdS/ Mo2C@C than that on CdS. The efficient separation of the photogenerated electron−hole pairs on CdS/Mo2C@C is further demonstrated by the much higher photocurrent on CdS/Mo2C@C than that on the pure CdS, as shown in Figure 5b. The enhanced charge separation on CdS/Mo2C@C may be due to the formation of the interface between the CdS nanoparticle and Mo2C@C nanosphere. It has been confirmed that, on CdS-based composite photocatalysts, the interface between CdS and other materials, such as TiO2 and carbon nitride, can efficiently promote the separation of the photogenerated electron−hole pairs formed on CdS.40−43 In addition, the pores with sizes ranging from 3.6 to 19.3 nm on the Mo2C@C nanosphere (Figure 3b) could also make some contributions to the more efficient charge separation on CdS/Mo2C@C, as the nanoscale pores may induce confinement effects favorable for charge separation and transfer.44,45 Visible-Light-Driven Photocatalytic Water Splitting. Visible-light-driven photocatalytic H2 evolution from water solution with lactic acid (10 vol %) as a hole scavenger was next performed on CdS/Mo2C@C and control samples (Figure 6). 5453

DOI: 10.1021/acssuschemeng.7b00787 ACS Sustainable Chem. Eng. 2017, 5, 5449−5456

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DISCUSSION

During the visible-light-driven photocatalytic H2 evolution from water on CdS/Mo2C@C, the CdS nanoparticles are responsible for the absorption of visible light.6−15,29,40,41 The zero H2 evolution on Mo2C@C (Figure S9) results from the absence of CdS. Mo 2 C is the cocatalysts for the photocatalytic reactions.26−32 The absence of Mo2C makes the H2 evolution rate on CdS/CNS (21.5 μmol h−1) much lower than that on CdS/Mo2C@C (554.3 μmol h−1). CdS/M200, in which M200 is Mo2C (Figure S1), exhibits a H2 evolution rate of 254.9 μmol h−1, which is lower than that on CdS/Mo2C@C, indicating that the carbon plays an important role during the photocatalytic reactions. The presence of carbon increases the surface area of the photocatalyst. The BET surface area of CdS/Mo2C@C is 276.1 m2 g−1, whereas that of CdS/M200 is only 67.5 m2 g−1. The larger surface area is a benefit for the interaction of the H2O molecules with the photocatalyst and thereby promotes the photocatalytic reaction. In addition, as shown in Figure 5b, the photocurrent on CdS/M200 is higher than that on the pure CdS nanoparticles but lower than that on CdS/Mo2C@C. This suggests that the separation of the photogenerated electron− hole pairs is efficiently improved due to the presence of carbon. As such, the carbon makes the surface area of the photocatalyst larger, enhances the charge separation and thus leads to a higher H2 evolution rate. For better understanding the role of Mo2C@C during the photocatalytic reactions on CdS/Mo2C@C, we prepared another two photocatalysts, denoted by CdS/Mo2C/C and CdS/C/Mo2C, respectively, for comparison. For fabricating CdS/Mo2C/C, a certain amount of CNS and 15 mg of CdS/ M200 were dispersed in 5 mL of water under ultrasonication for 30 min. After centrifuging, washing with DI water and ethanol for several times, and drying under vacuum at 25 °C for 12 h, the solid sample was used for the photocatalytic reaction. By fixing the CdS/M200 amount at 15 mg and changing the CNS amount used for preparing CdS/Mo2C/C, an optimum H2 evolution rate of 371.3 μmol h−1 was obtained (Figure S14 in the Supporting Information). CdS/C/Mo2C was fabricated by using the following method. A certain amount of M200 and 15 mg of CdS/CNS were dispersed in 5 mL of water under ultrasonication for 30 min, followed by centrifuging, washing with DI water and ethanol for several times, and drying under vacuum at 25 °C for 12 h. The solid sample after drying was applied for the photocatalytic reaction. By fixing the CdS/CNS amount at 15 mg and changing the M200 amount used for preparing CdS/C/Mo2C, an optimum H2 evolution rate of 337.1 μmol h−1 was achieved (Figure S15 in the Supporting Information). The photocatalytic activities of both CdS/Mo2C/C and CdS/C/Mo2C are lower than that of CdS/Mo2C@C. For CdS/Mo2C@C, the combination between Mo2C and carbon occurs during the hydrothermal and calcination processes, whereas for both CdS/Mo2C/C and CdS/C/Mo2C, the combination between Mo2C and carbon is physical. As such, the interaction between Mo2C and carbon on CdS/Mo2C@C could be more intimate than those on both CdS/Mo2C/C and CdS/C/Mo2C. This is favorable for the transfer of the photogenerated electrons, resulting in a higher H2 evolution rate on CdS/Mo2C@C. A possible mechanism for H2 evolution from the visible-lightdriven photocatalytic water splitting on CdS/Mo2C@C is illustrated in Figure 7. Visible light absorption by the CdS

Figure 7. Mechanism for the visible-light-driven photocatalytic H2 evolution from water on CdS/Mo2C@C.

nanoparticles generates electron−hole pairs. The photogenerated electron−hole pairs are then separated, followed by transfer of the electrons from the CdS nanoparticles to Mo2C@ C. Finally, water is split in the presence of the photogenerated electrons on Mo2C@C, producing H2. The photogenerated holes are consumed by the lactic acid molecules which are efficient scavengers of the holes.40,41 As shown by the adsorption−desorption measurements, there are pores, with size ranging from 3.6 to 19.3 nm, on Mo2C@C (Figure 3b). The pores could provide the ways for proton immigrates from the core to surface on Mo2C@C to fulfill the proton reduction reaction. The more efficient separation of the photogenerated electron−hole pairs on CdS/Mo2C@C could be the origin for the highly enhanced photocatalytic activity. Besides, the improved visible light absorption (Figure S8) could also make a contribution to the highly efficient H2 evolution on CdS/Mo2C@C. In summary, Mo2C@C nanospheres (∼200 nm) were fabricated and combined with CdS nanoparticles (∼5 nm) to form a noble-metal-free CdS/Mo2C@C photocatalyst for the visible-light-driven photocatalytic H2 evolution from water. Mo2C@C has a surface area of 268.4 m2 g−1, and pores with size ranging from 3.6 to 19.3 nm. CdS/Mo2C@C shows an enhanced visible-light-driven photocatalytic H2 evolution from water, with a rate of 554.3 μmol h−1 and a QE of 33.4%. An improved absorption of the visible light absorption and separation of the photogenerated electron−hole pairs are achieved on CdS/Mo2C@C. This could be responsible for the enhanced photocatalytic performance CdS/Mo2C@C. These findings are helpful for fabricating efficient noble-metal-free photocatalyst for visible-light-driven photocatalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00787. XRD patterns, TEM images, HRTEM of the CdS nanoparticles, XPS spectra of Mo 3d and C 1s on CdS/ Mo2C@C, UV−visible spectra, and photocatalytic activities (PDF) 5454

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.-X.P.). *E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (J.S.). ORCID

Yun-Xiang Pan: 0000-0002-7992-1544 Sen Xin: 0000-0002-0546-0626 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 21503062, U1662138, 21605102), and the State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, China. J.S. also thanks the Thousand Talents Program for Young Researchers of China for support.

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