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Facile Synthesis of Hierarchical CuMoS Hollow Sphere/Reduced Graphene Oxide Composites With Enhanced Photocatalytic Performance Ke Zhang, Yunxiang Lin, Changda Wang, Bo Yang, Shuangming Chen, Shuang Yang, Weiyu Xu, Haiping Chen, Wei Gan, Qi Fang, Guobin Zhang, Guang Li, and Li Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03767 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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The Journal of Physical Chemistry

Facile Synthesis of Hierarchical Cu2MoS4 Hollow Sphere/Reduced Graphene Oxide Composites With Enhanced Photocatalytic Performance Ke Zhang, †,§ Yunxiang Lin, †,§ Changda Wang, † Bo Yang, ‡ Shuangming Chen, † Shuang Yang, † Weiyu Xu, † Haiping Chen, † Wei Gan, † Qi Fang, † Guobin Zhang, † Guang Li, *, ‡ Li Song *, †

†

National Synchrotron Radiation Laboratory, CAS Center for Excellence in

Nanoscience, University of Science and Technology of China, Hefei 230029, China ‡

School of Physics and Materials Science, Anhui University, Hefei, China

1

ACS Paragon Plus Environment


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ABSTRACT: We present a controllable synthesis of ternary hierarchical hollow sphere, assembling by numerous particle-like Cu2MoS4, via a facile hydrothermal method. By adding graphene oxides (GO) in the reaction process, Cu2MoS4/reduced graphene oxide (RGO) heterostructures were obtained with enhanced photocurrent and photocatalytic performance. As demonstrated by electron microscopy observations and X-ray characterizations, considerable interfacial contact was achieved between hierarchical Cu2MoS4 hollow sphere and RGO, which could facilitate the separation of photo-induced electrons and holes within the hybrid structure. In comparison with the pure Cu2MoS4 hollow sphere, the obtained hybrid structures exhibited significantly enhanced light absorption property and the ability of suppressing the photo-induced electron-holes recombination, which led to significant enhancement in both photocurrent and efficiency of photocatalytic Methyl Orange (MO) degradation under visible light (λ > 420 nm) irradiation.

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INTRODUCTION

Since TiO2 photoelectrode is used to split water for generation of H2 and O2 by Fujishima and Honda, semiconductor-based photocatalysts have been attracting great attention for addressing global energy shortage and environmental problems during past decades.1-3 Various photocatalysts, such as sulphides,4 oxides,5 nitrides,6 have been successfully developed for highly efficient photocatalysis. In general, photocatalysis mainly comprises three steps: First, with irradiation of ultraviolet light or visible light, the electrons from the valence band (VB) realize the transition into the conduction band (CB), which form an equal number of holes at original location; second, photo-excited charges (electrons and holes) migrate to the surface of semiconductor; thirdly, the electrons and holes act as reductant and oxidant to realize dye degradation and water splitting. The efficiency of photocatalyst is mainly affected by these three steps. However, the most efficient catalysts for degradation pollutants and water splitting are noble metal-based catalysts, which are not allowed to be used for massive production because of their low abundance and high cost. Therefore, many catalysts have been made to substitute for noble metals in the last few years based on molybdenum, tungsten and copper.7-10 As we know, transition metal sulphides (TMSs) have widely attracted attention over recent years because of their excellent catalytic performance and extensive sources, such as WS2 and MoS2.11-13 It is well-known that catalytic performance is quite varied related to material’s structure, morphology and crystallinity. Up to now, efforts have been successfully made to improve the performance of catalysts, such as hybridizing heterostructures, 3



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elemental doping and structural engineering.14,

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Over the last few decades,

hierarchical hollow structures are widely prepared in catalysis, energy storage and energy conversion due to their unique structure-dependent properties, such as high active surface areas, low densities, and uniform size.16 Many hollow structures, like oxides and sulphides, have been prepared by hard, sacrificial or soft templates.17, 18 Generally, these hollow structures are much more active than their bulk materials. Lou et al. has prepared hierarchical MoS2 hollow-microboxes constructed by ultrathin nanosheets and it showed excellent electrocatalytic activity for hydrogen evolution reaction (HER).19 Ye et al. has prepared Ag2ZnGeO4 hollow spheres by ion-exchange reaction between Zn2GeO4 and Ag+, and it showed enhanced photocatalytic activity for dye degradation.20 On the other hand, a lot of strategies have been employed to enhance the photocatalytic performance by designing hybrid structures.21 Owing to its perfect charge transport properties, graphene or reduced graphene oxide (RGO) can be an ideal platform for hybrid structure, such as CdS/RGO, TiO2/RGO.22, 23 Tran et al has successfully prepared Cu2O/RGO composites, which have shown clear photocurrent and photostability enhancement compared with pure Cu2O.24 In short, RGO can be a solid state electron mediator in the hybrid structure, and it is also conducive to the stability and dispersion of photocatalysts. As a typical layered semiconductor, ternary Cu2MoS4 has been widely applied in photo(electro-)chemical catalysis.25 Cu2MoS4 has two phases: with space group P42m, I42m. The bandgap value of I-Cu2MoS4 is 1.71 eV, which means it shows good visible-light absorption.26 Meanwhile, Tran et al. successfully synthesized Cu2MoS4 4


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samples as novel electrocatalysts for hydrogen evolution reaction, due to its excellent catalytic performance.27 However, it’s still a challenge to prepare Cu2MoS4 samples with high surface area and high uniformity, such as hierarchical structures. Herein, we perform a facile and friendly hydrothermal method to prepare hierarchical Cu2MoS4 hollow spheres for the first time. Subsequently, a novel heterostructures hybridizing Cu2MoS4 hollow sphere and RGO was designed and fabricated, which showed both highly enhanced photocurrent and photocatalytic activity. We believe that these results will open a common method to synthesize Cu2MX4 (M=Mo, W; X=S, Se) materials with hierarchical hollow structures, and also provide reasonable strategy for exploring efficient catalysts to solve the problem of energy shortage and environment pollution. The presented hierarchical Cu2MoS4 hollow sphere/RGO was designed and synthesized using a hydrothermal method, which was shown in Scheme 1. First of all, Cu2O nanospheres and graphene oxide (GO) were prepared according to previous reports.28,

29

Then they were added to (NH4)2MoS4 aqueous solution. Due to

Kirkendall effect, the hierarchical structure can be prepared after several hours of reaction. Meanwhile, GO materials were reduced to RGO during this hydrothermal process. Interestingly, the Cu2MoS4 hollow spheres’ surface can be distinctly enwrapped with sheet-like RGO.

EXPERIMENTAL SECTION Sample Preparation. The synthesis process of GO and Cu2O nanosphere are shown in Supporting Information. 5%wt GO were added to 25 ml deionized water and 5



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ultrasonicated for 3 h to get completely dispersed GO solution. After that, (NH4)2MoS4 powder (36.4 mg) and 20 mg Cu2O nanosphere was added to the above solution (containing 10 mL DI water) and stirred for 30 min. The precursor was stirred for 10 min, then the solution was transferred into a Teflon-lined stainless steel autoclave and heated at 200 °C. The temperature was held for 24 h and then naturally cooled down to room temperature. The resulting samples were separated by centrifuging and washed several times with deionized water and ethanol to remove any possible impurity, and then dried at 60 °C under vacuum for 6 hours. Without adding GO in the chemical process, the pure Cu2MoS4 hollow sphere was obtained. Sample Characterization. X-ray powder diffraction (XRD) was used to analyze the sample’s crystal structure via PANalytical diffractometer (radiated wavelength λCu Kα1

= 1.54056 Å). The elements’ chemical condition and composition was tested by

X-ray photoelectron spectroscopy (XPS, ThermoESCALAB250). Scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, JEOL JEM-2100F) were carried out to characterize the morphologies and structures of the samples. The visible light absorption of the photocatalysts was checked by UV-vis spectra (Shimadzu DUV-3700 spectrophotometer). After 12 hours degassing for the samples under vacuum, N2 adsorption-desorption were recorded via Micromeritics Tristar 3020, and Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area. The photoluminescence spectra of the samples were recorded by Fluorolog-3-Tau Spectrofluorometer (JY Horiba). Photocurrent Measurements. The electrochemical station (CHI 660D) was 6


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employed with three-electrode cell configuration (a working electrode, a Pt foil counter electrode, an Ag/AgCl referenced electrode). The 0.5 M Na2SO4 solution was selected as electrolyte after purged with Ar for 30 min. A Xe lamp with 300 W (Perfect Light, China) was used as light source. A cut-off filter (λ < 400 nm) was used. The working electrode was prepared by dip-coating process. Typically, 1 mL ethanol was used to disperse 10 mg Cu2MoS4 samples. The mixed samples were deposited onto a glass substrate with indium tin oxide (ITO) coated (2.0 cm × 2.0 cm) by a spin coater (SC-1B, China). The samples were heated at 60 °C via a vacuum oven for 0.5 h. the photocurrent densities were tested under chopped light irradiation (the time for light on/off cycles is 20 s) with a bias potential of 0.8 V vs. Ag/AgCl for 180 s at room temperature. Photocatalytic Performance. Methylene orange (MO) was used as a typical pollution to check the photocatalytic activities of the Cu2MoS4 samples. Typically, 50 mg photocatalyst were added to a 100 mL of aqueous solution of MO (15 mg·L-1) in a reactor (200 mL), and then stirred in the dark for 120 min to ensure adsorption/desorption equilibrium between Cu2MoS4 catalysts and dye. After that, a 300 W Xe lamp with UV cutoff filter (providing visible light with a wavelength longer than 420 nm) was switched on to realize the degradation of MO dye. At different intervals, 5 mL of photoreaction suspension was removed. Before the spectroscopy measurement, the photocatalyst were removed from the mixture solution by centrifugation. Blank experiment without photocatalyst was also done. . 7



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RESULTS AND DISCUSSION

Morphology and Structure Analysis. Cu2O nanosphere are used as template for the synthesis of Cu2MoS4 hollow structure shown in Figure S1 (Supporting Information). Figure 1 reveals the SEM and TEM analysis results carried out on the as-synthesized Cu2MoS4 hollow sphere. It is observed that spheres-like samples with size about 300-400 nm have been achieved (Figure 1A, 1B and 1C) with high uniformity and no impurity. Obviously, it can be seen that the hollow structures are composed of Cu2MoS4 nanoparticles (10-20 nm) and shows rough surfaces, which are believed to have a very high surface area. HRTEM analysis (Figure 1D) on a single hollow sphere reveals that the nanoparticles are well crystalline and exposes its (001) face with lattice distance of 5.1 Å. To the best of our knowledge, these ternary Cu2MoS4 samples with unique hierarchical hollow structure are synthesized for the first time in comparison with previous research. Figure 1E and 1F show the SEM and TEM images of Cu2MoS4/RGO composites. The red arrows in Figure 1F means that the considerable interfacial contact between Cu2MoS4 hollow sphere and RGO is formed, which could promote efficient separation and transfer of photo-induced electrons and holes within the hybrid structure. The crystal structure of the as-prepared Cu2MoS4 samples are characterized by powder X-ray diffraction (XRD). As shown in Figure 2A, the pure Cu2MoS4 sample is obtained by hydrothermal method without adding GO. As for the pattern of Cu2MoS4/RGO, there is no difference with pure Cu2MoS4. All of the peaks could be indexed to the Cu2MoS4 shown in Figure S2 (Supporting Information). The diffraction 8


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peak of RGO is not obvious in this XRD data, indicating the low RGO weight in the composites. Raman spectroscopy was further used to confirm the presence and quality of the prepared Cu2MoS4/RGO. The Raman spectra of pure GO and Cu2MoS4/RGO is shown in Figure 2B. The two characteristic bands at 1350 cm-1 (D band) and 1598 cm-1 (G band) are obvious, which strongly implies the existence of RGO.31 Compared with pristine GO, Cu2MoS4/RGO shows a slight increase in ID/IG ratio (1.14 vs 0.93), which suggests the transformation of GO to RGO during the reaction process.32 X-ray photoelectron spectroscopy (XPS) is used to characterize the reduction effectiveness of GO. The high resolution XPS for C 1s is shown in Fig 3A and 3B. As for GO, there are three peaks displayed at 284.7, 286.8, 288.9 eV, which were assigned to C=C, C-O, C=O.33 It is obviously observed that the intensity of C-O and C=O in Cu2MoS4/RGO composites is much lower than those of GO, which suggests GO is sufficiently reduced to RGO in the reaction process. This result is consistent with the Raman analysis. The UV-vis diffuse reflectance spectra for both Cu2MoS4 hollow sphere and composites are shown in Figure 4A. The composites display stronger and wider absorption in the range of 400-800 nm compared with the hollow sphere samples. According to Tauc’s formulation,34 the optical band gap of pure Cu2MoS4 can be calculated at around 1.74 eV, which was shown in Figure S3 (Supporting Information). The inset of Figure 4A is an image of the two samples, which are dispersed in absolute ethanol. The composites’ solution exhibits dark- brown color, while the pure Cu2MoS4 shows brown-red. Photoluminescence (PL) emission spectroscopy are 9



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employed to investigate the efficiency of Cu2MoS4 hollow sphere and Cu2MoS4/RGO composites in charge separation. As shown in Figure 4B, the lower PL intensity of Cu2MoS4/RGO composites suggests the significant charge separation in contrast to the case of Cu2MoS4 hollow sphere. As is known to all, RGO has good charge transport properties. In the hybrid nanocomposites, it is suggesting that Cu2MoS4 and RGO can act as electron donor and electron acceptor, respectively. Photocurrent and Photocatalytic Activities. The stronger absorption and higher charge separation efficiency of Cu2MoS4/RGO composites indicate that it may exhibit better performance in photocurrent and photocatalytic activities than the single composition. Photocurrent measurements under visible-light (λ>420 nm) irradiation of Cu2MoS4 hollow sphere and composites are investigated, as shown in Figure 5A. It is worth noting that the photocurrent is negligible when the illumination is stopped. Obviously, the Cu2MoS4 composites show much larger photocurrent than pure Cu2MoS4 (10.5 µA•cm-2 vs 2.0 µA•cm-2), implying the highly efficiency of charge separation

in

Cu2MoS4/RGO

hybrid.

This

result

agrees

well

with

the

photoluminescence emission spectroscopy. In addition, the photocatalytic performance of the two synthetic samples were further verified through methyl orange (MO) photodegradation under visible-light illumination (λ>420 nm) in aqueous solution. Without catalysts, there was less than 3% MO degraded under visible light, which is shown in Figure 5B. However, in the presence of Cu2MoS4/RGO, the MO-degradation efficiency can reach up to 95% after

10


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75 min, but only less than 55% of MO molecules are decomposed with pure Cu2MoS4 hollow sphere in the same period. As we know, large surface area, suitable bandgap and good monodispersity are beneficial to the performance of photocatalysts.35 In the designed Cu2MoS4/RGO heterostructures, the hollow spheres are composed of nanoparticles which expose more surfaces and edges to contact with MO molecular. Meanwhile, Cu2MoS4 hollow spheres dispersed well on RGO, which could prevent catalysts from large agglomeration. This is confirmed by our Brunauer-Emmett-Teller data, as shown in Figure 6. The Cu2MoS4/RGO heterostructures show almost 3 times larger BET surface area than that of pure Cu2MoS4 hollow sphere (33.53 m2g-1 vs 11.46 m2g-1). Based on these results, the mechanism of photocatalysis is proposed in scheme 2. Under visible-light irradiation, the hybrid structure shows enhanced light absorption, which suggests more solar energy could be utilized and converted into chemical energy. Moreover, electronic interactions and charge equilibration between RGO and Cu2MoS4 led to the photoexcited electrons migrate to the RGO. Meantime, several photoexcited holes are left at the surface of Cu2MoS4. This process could efficiently inhibit the recombination of photo-generated charges. The larger BET surface area could contribute to the attachment of Cu2MoS4/RGO heterostructures and MO molecule, and finally realizes efficiently decompose of organic pollution.

Conclusions In summary, based on a facile hydrothermal synthesis method, highly efficient 11



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visible-light-driven

photocatalyst

based

on

hierarchical

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Cu2MoS4

hollow

sphere/reduced graphene oxide composites have been successfully investigated. Owing to the novel hierarchical structures between hollow sphere and RGO, large enhancement in visible-light absorption behavior and highly efficient charge separation were achieved in Cu2MoS4/RGO heterostructures. As a result, the obtained hybrid showed excellent photocurrent and photocatalytic activity compared with pure Cu2MoS4 hollow spheres. This work opens a way for constructing hierarchical TMSs architecture photocatalysts for water pollutant treatment, photocatalytic water splitting, solar cells or other potential applications. Supporting Information The synthetic process of graphene oxide and cuprous oxide. XRD patterns of pure Cu2MoS4 samples. The plot of [αhv]2 versus hv for calculating the optical energy gap. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected]; Telephone number: (+86) 0551 63861867 * Email: [email protected]; Telephone number: (+86) 0551 63602102

Author Contributions §

These authors contribute equally

Notes The authors declare no competing financial interest.

Acknowledgement. This work was financially supported by the National Basic 12


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Research Program of China (2014CB848900), the National Natural Science Foundation of China (U1232131, U1532112, 11375198, 11574280), and the Fundamental Research Funds for the Central Universities (WK2310000053), User with Potential from CAS Hefei Science Center (2015HSC-UP020) and funds from Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) Nankai University. We also thank the Shanghai synchrotron Radiation Facility (14W1, SSRF), the Beijing Synchrotron Radiation Facility (1W1B and soft-X-ray end station, BSRF), the Hefei Synchrotron Radiation Facility (MCD, photoemission and Catalysis/Surface Science Endstations, NSRL) and the USTC Center for Micro and Nanoscale Research and Fabrication for help in characterizations. The authors also thank Dr. Yu Li, Dr. Jian Yang for useful discussions.

FIGURES 13



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Scheme 1. Schematic illustration of the formation of Cu2MoS4/RGO composites via a hydrothermal method.

Figure 1. A, B) SEM images of Cu2MoS4 hollow sphere at different magnifications; C, D) TEM and HRTEM image of Cu2MoS4 hollow sphere; E, F) SEM and TEM images of Cu2MoS4 /RGO composites. Scale bar in A is 1µm, in B and C are 100 nm, in D is 5 nm, in E is 500 nm, and in F is 200 nm.

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Figure 2. A) XRD analysis for as-prepared Cu2MoS4 hollow sphere and Cu2MoS4/RGO composites. B) Raman spectra analysis for GO and Cu2MoS4/RGO, respectively.

Figure 3 XPS spectra of GO (A) and RGO (B) in the C 1s region.

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Figure 4. A) UV-vis diffuse reflectance spectra of Cu2MoS4 and Cu2MoS4/RGO, the inset is a photograph of Cu2MoS4 and Cu2MoS4/RGO dispersed in ethanol. B) Photoluminescence spectra of the Cu2MoS4 and Cu2MoS4/RGO, the excited laser wavelength is 325 nm.

Figure 5. A) Photocurrent response of pure Cu2MoS4 hollow spheres and Cu2MoS4/RGO heterostructures; B) MO degradation over pure Cu2MoS4 and Cu2MoS4/RGO under visible light.

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Figure 6. The nitrogen adsorption–desorption isotherms of Cu2MoS4/RGO (upside) and hollow sphere (bottom) show that the BET surface areas are about 33.53 m2g−1 and 11.46 m2g−1, respectively.

Scheme 2.

Schematic illustration of the charge transfer and separation of

Cu2MoS4/RGO for photodegradation pollutants under visible light.

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