One-pot Synthesis of CdS Irregular Nanospheres Hybridized with

Jun 13, 2017 - Large volumes of hydrogen bubbles were generated within only 2 s as the photocatalysis started, as demonstrated by the photocatalytic v...
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One-pot Synthesis of CdS Irregular Nanospheres Hybridized with Oxygen-Incorporated Defect-Rich MoS2 Ultrathin Nanosheets for Efficient Photocatalytic Hydrogen Evolution Shouwei Zhang,† Hongcen Yang,† Huihui Gao, Ruya Cao, Jinzhao Huang,* and Xijin Xu* School of Physics and Technology, University of Jinan, Shandong 250022, PR China S Supporting Information *

ABSTRACT: Robust and highly active photocatalysts, CdS@ MoS2, for hydrogen evolution were successfully fabricated by one-step growth of oxygen-incorporated defect-rich MoS2 ultrathin nanosheets on the surfaces of CdS with irregular fissures. Under optimized experimental conditions, the CdS@ MoS2 displayed a quantum yield of ∼24.2% at 420 nm and the maximum H2 generation rate of ∼17203.7 umol/g/h using Na2S−Na2SO3 as sacrificial agents (λ ≥ 420 nm), which is ∼47.3 and 14.7 times higher than CdS (∼363.8 μmol/g/h) and 3 wt % Pt/CdS (∼1173.2 μmol/g/h), respectively, and far exceeds all previous hydrogen evolution reaction photocatalysts with MoS2 as co-catalysts using Na2S−Na2SO3 as sacrificial agents. Large volumes of hydrogen bubbles were generated within only 2 s as the photocatalysis started, as demonstrated by the photocatalytic video. The high hydrogen evolution activity is attributed to several merits: (1) the intimate heterojunctions formed between the MoS2 and CdS can effectively enhance the charge transfer ability and retard the recombination of electron−hole pairs; and (2) the defects in the MoS2 provide additional active S atoms on the exposed edge sites, and the incorporation of O reduces the energy barrier for H2 evolution and increases the electric conductivity of the MoS2. Considering its low cost and high efficiency, this highly efficient hybrid photocatalysts would have great potential in energy-generation and environment-restoration fields. KEYWORDS: intimate heterojunctions, CdS irregular nanospheres, oxygen-incorporated defect-rich MoS2 ultrathin nanosheets, noble-metal-free, water splitting

1. INTRODUCTION

To address this issue, loading of co-catalysts on semiconductor photocatalysts not only facilitate in accelerating photogenerated charge separation but also supply a large number of active sites for hydrogen generation, facilitating the hydrogen evolution reaction rate and stability.3,8,16−20 The lowcost two-dimensional MoS2 has attracted many attentions in electrocatalytic hydrogen evolution with the goal to replace the expensive Pt-based noble metals.21−26 Moreover, many reports demonstrated that MoS2 could act as an effective co-catalyst to achieve improved activity toward the photocatalytic hydrogen evolution and organic pollutant degradation under visible light irradiation.7,10,13,27−30 For example, the TiO2@MoS2 heterostructure showed a maximum hydrogen production rate of 1.6 mmol/g/h and excellent organic dye degradation performance.27 The enormous theoretical and experimental results had proven that unsaturated S atoms located along the edges of the MoS2 layers were catalytically active for hydrogen evolution, while the S atoms on the basal plane had no activity.31 Therefore, optimally nanosized MoS2 with more exposed edge

Converting solar energy into chemical fuels is attracting great interest for the resolution of the energy crisis.1 Since the 1970s, the discovery of TiO2-supporting Pt electrodes for hydrogen evolution by Fujishima and Honda has induced photochemical water splitting to generate hydrogen by utilizing a photocatalyst to become the main trend for hydrogen-energy production.2 As the large band gap of TiO2 (∼3.2 eV) greatly inhibits its potential application,3,4 many newly designed and constructed photocatalysis systems based on semiconductor materials such as metal oxides, metal chalcogenides, and metal oxynitrides, etc. were explored to make full use of the major proportion of solar light.5−11 Among them, CdS gradually becomes the promising one because of its appropriate band gap (∼2.4 eV) for effective visible light absorption and suitable valence-band and conduction-band levels for the hydrogen evolution reaction.12−14 However, the high photogenerated charge-recombination rate and photocorrosion seriously impair the photocatalytic efficiency of CdS.15 Therefore, in developing effective strategies, novel surface structures with which to inhibit the charge recombination are critical for the application of CdSbased photocatalysts in water splitting. © 2017 American Chemical Society

Received: March 14, 2017 Accepted: June 13, 2017 Published: June 13, 2017 23635

DOI: 10.1021/acsami.7b03673 ACS Appl. Mater. Interfaces 2017, 9, 23635−23646

Research Article

ACS Applied Materials & Interfaces

nanoparticles coated with defect-rich MoS2 nanosheets, in which the amount of active-edge S atoms of MoS2 nanosheets were covered by CdS, which is unfavorable for the promotion of the photocatalytic reactions. From the above discussion, the core−shell-structured MoS2/CdS composed of an outer shell of numerous oxygen-incorporated defect-rich MoS2 nanosheets and an inner CdS core is suitable for photocatalytic H2 evolution. Herein, we demonstrate a simple strategy for the fabrication of a CdS@MoS2 core−shell composite with rough surface and irregular fissures via a one-step hydrothermal process. The synthesized photocatalysts were tested for photocatalytic hydrogen evolution under visible-light irradiation by scarifying Na2S and Na2SO3 as sacrificial agents under optimized conditions. The CdS irregular nanospheres with rough surface and irregular fissures provide large and intimate interfacial contact between the ultrathin MoS2 shell and CdS core. This unique structure not only can provide abundant reactive sites for hydrogen evolution but also can increase photogenerated charge transfer and reduce the recombination probability of photogenerated charge. Thus, this work not only showed a simple ultrathin nanosheet coating strategy for the controllable aqueous-phase synthesis of highly efficient CdS@MoS2 core− shell composite with improved photocatalytic activity but also highlights the prospective scope to develop the potential applications of core−shell nanocomposite in photocatalytic hydrogen evolution for energy supply.

sites and active S atoms is proposed to deliver higher cocatalytic activities for CdS-based photocatalysts.10,31,32 Until now, although many exciting advances in the synthesis of CdS/MoS2 hybrid heterojunctions have been achieved, there still exist several problems restricting the further enhancement of their photocatalytic activity.15,33−35 First, the aggregated or large CdS nanoparticles significantly lower the surface-tovolume ratio of CdS and increase the diffusion length of photogenerated electrons and holes, thus leading to a decrease of photocatalytic activity.36−38 Second, regular morphology of CdS (like nanowires) with a high degree of exposed MoS2 surface causes an insufficient exploitation of the advantages of the 2D nanostructures of MoS2, and the synergistic interactions between CdS and MoS2 should be further strengthened. Third, MoS2/CdS hybrids with various nanostructures, such as nanowires, nanobelts, and nanorods, and nanospheres have been prepared, while these processes require two or more complicated synthetic steps, which significantly limit the industrialized enlargement of the hybrids.30,39,40 Fourthly, most of the current methods for the fabrication of MoS2/ CdS hybrids require environmentally harmful surfactants (such as polyvinylpyrrolidone), toxic organic compounds (such as ethylenediamine), or mixed-phase solvents, which significantly increase the cost and are harmful to the environment. Therefore, it is urgently desirable to develop a facile synthetic approach for fabricating MoS2@CdS hybrids with optimized nanoarchitecture as high-performance photocatalysts.41,42 The photocatalytic H2 evolution involves in three processes: (1) absorption of light by a semiconductor to generate electron−hole pairs, (2) charge separation and migration to the surface of the semiconductor, and (3) surface reactions at catalytic sites corresponding to H2 evolution. The rational design of a high-performance CdS/MoS2 photocatalyst requires at least (1) accelerating electron transfer from CdS to MoS2 cocatalyst and reducing surface charge recombination and (2) enriching the active sites on MoS2 co-catalyst and minimizing charge recombination in MoS2. The architecture and active sites of photocatalysts play a critical role in obtaining outstanding properties.43−45 Therefore, the more active sites and higher conductivity of photocatalysts play a critical role in hydrogen evolution reaction. For MoS2, the unsaturated S atoms on the exposed edges of nanosheets are the active sites for H2 generation. According to the density functional theory (DFT) calculations and experiments by Xie J. et al.,21 compared with the pristine MoS2 system, (1) defect-rich structure provides the opportunity to tune the number of active sites in H2 evolution reaction, generating more unsaturated S atoms in a more-disordered structure, providing more effective sites for H2 evolution reaction; and (2) elemental incorporation has been widely used to regulate the electronic structure of semiconductors and thus tune the conductivity of materials, giving the opportunity to modulate the intrinsic conductivity of MoS2 nanosheets, facilitating the H2 generation. Consequently, for the CdS@MoS2, the grown unsaturated S atoms on the exposed edges of MoS2 nanosheets with O incorporation may have had a lower energy barrier for H2 evolution and a higher electric conductivity compared with the pristine counterpart, enhancing their photocatalytic capabilities. However, for most of the reported MoS2/CdS systems, there are two main types of MoS2 used as co-catalysts. One is defect-free MoS2 nanosheets, which cannot provide enough active sites for H2 evolution. The other is defect-rich MoS2 nanosheets. Although it can provide a large number of active sites, the products were CdS

2. EXPERIMENTAL SECTION 2.1. Preparation of CdS@MoS2, CdS, and MoS2. The CdS@ MoS2 core−shell composite were prepared by a hydrothermal method. Typically, 2.4 mmol of Cd(CH3COO)2·2H2O, 0.12 mmol Na2MoO4· 2H2O, and 36 mmol of thiourea were dissolved in 60 mL of deionized water at room temperature and then transferred into a 100 mL Teflonlined stainless steel autoclave. The autoclave was heated to 200 °C for 24 h and cooled naturally to room temperature, and the products were washed with deionized water and ethanol several times. The MoS2 loading can be easily adjusted by changing the amount of Na2MoO4·2H2O. A series of CdS@MoS2 core−shell composite with Mo-to-Cd molar ratios of 0, 2%, 5%, 10%, 15%, and 25% were prepared. As a control, free CdS and MoS2 were also synthesized using the same procedure as mentioned above without the addition of Na2MoO4·2H2O or Cd(CH3COO)2·2H2O. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements revealed that the real content of MoS2 in each CdS@MoS2 composite approaches its theoretical loading (Table S1). 2.2. Characterization. Powder X-ray diffraction (XRD) data were collected using a D/MAX2500 V diffractometer using Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB250 with Mg Kα as the source and the C 1s peak at 284.6 eV as an internal standard. The morphologies and compositions were characterized using a JEOL JSM-6330F scanning electron microscopy (SEM) instrument operated at a JEOL-2100 field emission transmission electron microscope (FETEM) at an accelerating voltage of 200 kV. UV−vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-2500 spectrophotometer using BaSO4 as the reference. Photoluminescence (PL) spectra were obtained on a FLUOROLOG-3-TAU. All of the spectra were taken at room temperature with an excitation wavelength of 365 nm. The Cd and Mo content in composites were measured using ICP-AES (ICPE9000, Shimadzu). Photocurrent measurements (i−t curves) were conducted in three-electrode cell system by using a CHI660E electrochemical station at a 0.5 V potential bias under a 300 W Xe arc lamp as light source. The cleaned ITO glass deposited with photocatalysts, Pt flake, and a saturated calomel electrode were, respectively, used as working electrodes, counter electrode, and 23636

DOI: 10.1021/acsami.7b03673 ACS Appl. Mater. Interfaces 2017, 9, 23635−23646

Research Article

ACS Applied Materials & Interfaces reference electrode. Na2SO4 (0.5 M) was used as the electrolyte. Electrochemical impedance spectroscopy (EIS) was performed in a dummy cell with a computer-controlled Autolab impedance measurement unit. EIS was determined over the frequency range of 0.01−105 Hz with an AC amplitude of 10 mV at the open-circuit voltage. 2.3. Evaluation of Photocatalytic Activity. The photocatalytic hydrogen evolution experiments were performed in a 500 mL Pyrex glass cell. A 300 W Xe lamp was used as the visible light source equipped with a 420 nm cutoff optical filter. All experiments were performed in triplicate. Average and standard deviations were calculated based on the triplicate results and the relative errors were about 5%. In general, the CdS@MoS2 photocatalyst (50 mg) was dispersed by a constant stirring in mixed aqueous solution containing 0.25 M Na2S and 0.35 M Na2SO3 (100 mL) served as sacrificial agents. A continuous magnetic stirrer was applied to keep the photocatalyst in suspension status during the whole experiment. The system was kept at 6 ± 0.5 °C. Before irradiation, the reaction system was pumped to vacuum and ensured the reaction system in an anaerobic condition. The H2−solar system (Beijing Aulight CO., Ltd.) with a gas chromatogram (GC), equipped with a thermal conductivity detector (TCD), TDX-01 column, and Ar as carrier gas, was used to collect and online-detect evolved H2. All glasswares were carefully rinsed with distilled water prior to use. The H2 evolution rate on CdS@MoS2 was measured with different irradiation wavelengths, band-pass filters of 420 ± 5, 450 ± 5, 475 ± 5, 500 ± 5, and 520 ± 5 nm were used for the reaction. The quantum yield of the catalyst for H2 evolution was measured by applying a Xe lamp (300 W) with a 420 ± 5 nm bandpass filter. The H2 yield of 1 h of photoreaction was measured. The quantum yield value was calculated according to the following equation:17 quantum yield = = = =

Figure 1. XRD patterns of CdS and CdS@MoS2 composites.

number of reacted electrons × 100% number of incident photons 2 × number of evolved H 2 molecules × 100% number of incident photons 2 × NH2 Ni 2 × NH2 I×A×t×λ h×c

× 100% × 100% Figure 2. UV−vis DRS of CdS and CdS@MoS2 composites.

where N is the number of electrons, photons, or molecules, I = 5.54 mW/cm2, A = 26.4 cm2, t = 3600 s, λ = 420 nm, h = 6.62 × 1034J·s, and c = 3.0 × 108 m/s.

composites showed an increase over the visible light region from 520 to 700 nm with respect to CdS; meanwhile, the absorption of CdS@MoS2 composites was enhanced with the increase of MoS2 contents. Moreover, the Eg values of pure CdS and CdS@MoS2-2% to CdS@MoS2-25% were ∼2.21, ∼ 2.20, ∼ 2.18, ∼ 2.18, ∼ 2.19, and ∼2.18 eV, respectively, indicating that the introduction of MoS2 hardly causes the inherent band structure of the CdS but significantly increased the visible light absorption capability.48 This further corroborates even distribution of MoS2 coated on the surface of CdS irregular nanosphere rather than entering the crystal grating of CdS. The pore structures and BET surface areas of the CdS and CdS@MoS2-5% were measured by N2 adsorption−desorption measurements at 77.4 K and shown in Figure S1. The BET surface area of CdS@MoS2-5% was calculated to be ∼17.35 m2/g, which was almost 5.06 times higher than that of CdS (∼3.43 m2/g). The pore-size distribution curves indicated that the CdS and CdS@MoS2-5% were rich in pores of sizes between 1 and 20 nm, which proved that the micro- and mesoporous structures were well-developed in CdS and CdS@ MoS2-5%.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. The powder XRD patterns of pure CdS and a series of CdS@MoS2 composites were displayed in Figure 1. Strong diffraction peaks observed at 25.02°, 26.66°, 28.35°, 36.75°, 43.91°, 48.05°, 51.13°, 52.02°, and 53.05° corresponded to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, respectively. These diffraction peaks can be well-assigned to the hexagonal CdS phase (JCPDS card no. 02-0549).15,39,46,47 No diffraction peaks for MoS2 were detected in CdS@MoS2 composites, even for CdS@MoS2 with 25 wt % MoS2, suggesting amorphous (or low-crystallinity) qualities and high dispersity of MoS2 in the CdS@MoS2 composites, which is beneficial as a photocatalyst.15,48 The optical properties of CdS and CdS@MoS2 composites were analyzed by UV−vis measurements, as depicted in Figure 2. The band edge positions of CdS and CdS@MoS2 composites were at ∼560 nm.34,49 For pure MoS2, a humped peak is revealed, located at about ∼450−700 nm, which is the result of a wide absorption in the visible light region. Interesting, the 23637

DOI: 10.1021/acsami.7b03673 ACS Appl. Mater. Interfaces 2017, 9, 23635−23646

Research Article

ACS Applied Materials & Interfaces

Figure 3. XPS of pure CdS, MoS2, and CdS@MoS2-5% composite: (A) survey spectra, (B) Cd 3d spectra, (C, D) Mo 3d spectra, (E, F) O 2s spectra, and (G, H) S 2p spectra.

that the oxygen is incorporated into CdS@MoS2, which is consistent with the O 1s results.30 The O 1s peaks can be deconvoluted into three peaks. The peak located at 532.18 and 530.98 eV can be attributed to adsorbed water and −OH, respectively. The peak located at 530.03 eV is corresponding to the binding energy of oxygen in MoIV−O, suggesting the existence of MoIV−O bonds. These results were agreed well with the Xie and Wang groups’ analyses, respectively.21,51,52 After the hybridization of the MoS2 nanosheets with CdS nanospheres, the Cd 3d peaks in CdS@MoS2-5% exhibited a blue shift of ∼0.3 eV with respect to that for individual CdS (Figure 3A). In contrast, the binding energies of Mo 3d in CdS@MoS2-5% exhibited an apparent red shift of ∼0.3 eV compared with that for individual MoS2 (Figure 3C,D). These phenomena can be explained by partial electron transfers from CdS to MoS2, i.e., increasing (decreasing) the electron density of MoS2 (CdS) leads to the reduction (enhancement) of the

The chemical compositions and elemental valence states were further investigated by XPS. The survey spectrum clearly indicated the existence of Cd, Mo, S, and O with trace amount of C element (Figure 3A). For pure CdS, the Cd 3d5/2 and 3d3/2 peaks were located at ∼405.08 and 411.88 eV, respectively, confirming the +2 oxidation state of Cd in CdS (Figure 3B).12,50 After coupling with MoS2, the Cd 3d peaks shifted to ∼405.38 (3d5/2) and 412.18 (3d3/2) eV, respectively. While XPS is a surface-sensitive technique to analyze the surface elements of the materials within 10 nm, the detection of the Cd from CdS in the CdS@MoS2-5% can be ascribed to the ultrathin and discontinuous growth of nanosheets (