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Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires
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Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires Chunyang Zhai a, Mingjuan Sun a, Mingshan Zhu a,*, Ke Zhang b, Yukou Du b a b
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, PR China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
In this paper, a new and high-performance visible-light-activated electrode is designed in
Received 26 August 2016
terms of two dimensional (2D) MoS2 nanosheets enwrapped 1D CdS nanowires as photo-
Received in revised form
activated support for depositing Pt nanoclusters. Compare to traditional ambient electro-
29 October 2016
catalytic process, the as-prepared photo-activated electrode shows evidently improved
Accepted 4 November 2016
electrocatalytic activity and stability of methanol oxidation reaction (MOR) under visible
Available online xxx
light illumination. The efficient interfacial electron transfer from the excited CdS moieties
Keywords:
and electrocatalytic process for the boosting of catalytic efficiency.
to the decorated ultrathin MoS2 shell contributes to the synergistic effect of photocatalytic Methanol oxidation
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Molybdenum disulfide Cadmium sulfide Visible light
Introduction Fuel cell, as a technology for direct conversion of the chemical energy of a fuel into electrical energy, has been recognized as a promising future power source [1]. Recently, traditional noble metal anode electrocatalysts hybridized with optically active semiconductors have been certified as the promising photoelectrocatalysts for the improvement of fuel cell anodic reactions [2e12]. With assistance of light illumination, those of noble metal/semiconductor modified electrodes displayed evidently boosted catalytic performance and stability for the
anodic oxidation of small organic molecules (SOMs). Despite some efforts have been devoted to the study of photoassisted fuel cell, most of optically active semiconductors have localized in UV-light-activated TiO2, which might hinder their broad applications. Multifarious nanostructures with different dimensions including zero-, one-, two-, and three-dimensional (0D, 1D, 2D and 3D), have received wide recognition for their unique sizee and shapeedependent physicochemical properties [13e16]. Among these nanostructures, well-defined 1D nanoarchitectures such as wires, rods, belts, etc. have become the focus of intensive research owing to their unique large aspect
* Corresponding author. E-mail address: [email protected] (M. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2016.11.035 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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ratio, which is beneficial to a directional charge transfer with a reduced grain boundary scattering, since the discovery of carbon nanotubes by Iijima [14e20]. Nowadays, the 1D nanomaterials play important roles as key units into nanoscale optic, electronic, electrochemical and particular interest to photo-/electro-catalytic fields [14,15,19,20]. 1D CdS nanomaterials are fascinating and promising optoelectronical material for the energy conversion [19,20]. This is because of its unique and versatile fundamental properties including desired band-gap width (2.4 eV), relative low work function, high refraction index, excellent transport properties, high electronic mobility, etc. However, the pure CdS suffers from the rapid recombination of photogenerated electrons and holes, and also severe photocorrosion effect. Recently, molybdenum sulfide (MoS2) nanosheet, a 2D chalcogenide nanomaterial with the same hexagonal crystalline structure as CdS nanoparticles (NPs), is often used to improve the photocatalytic performance of CdS nanostructures [21e31]. Exhibiting p-type properties, the 2D MoS2 nanosheets with large surface area can be integrated with an n-type CdS semiconductor to improve the separation of photoinduced electrons and holes. For example, Li's group had reported that MoS2 as a cocatalyst loaded on CdS NPs for the enhancement of photocatalytic H2 evolution under visible light irradiation [22,23]. Zhang et al. synthesized 2D MoS2/CdS pen nanohybrids through a one-pot solvothermal process for photocatalytic H2 production [25]. However, the above investigations limited CdS NPs modified MoS2 sheets, we expect that it is necessary to construct a core/shell nanoarchitecture with 1D CdS as core and 2D MoS2 sheets as shell for the investigation of charge separation and interfacial electron transfer of CdS/MoS2. Moreover, compared to massive studies of high-efficiency photocatalytic H2 evolution on the CdS/MoS2 heterostructure, comparatively few reports have been published on this hybrid for other applications. Herein, we reported the fabrication of a novel and highperformance visible-light-activated electrode in terms of 2D MoS2 nanosheets enwrapped 1D CdS nanowires. Firstly, by means of a one-pot solvothermal method, core/shell heterostructures with ultrathin 2D MoS2 sheets coated 1D CdS nanowires were easily obtained (Scheme 1). Then, the asprepared heterostructures can be served as support for depositing Pt nanoclusters. Interestingly, compared to traditional methanol oxidation reaction (MOR), the ternary as-prepared Pt-CdS/MoS2 photoanode showed evidently improved electrocatalytic activity and stability of MOR under visible-light illumination. The highly efficient interfacial
charger transfer from excited CdS moieties to decorated ultrathin MoS2 shell contributes to the synergistic effect of photocatalytic and electrocatalytic process for the boosting of catalytic methanol oxidation. The outstanding catalytic performance suggests that the CdS nanowire/MoS2 nanosheet core/shell heterarchitecture could act as a promising photoactivated material in solar and chemical energy conversion.
Experimental Materials Thiourea (CH4N2S), cadmium acetate dihydrate (C4H6CdO4$2H2O), molybdenum (II) acetate dimer (C8H12Mo2O8), H2PtCl6$6H2O, and ethylenediamine (C2H8N2) were purchased from Sinopharm Chemical Reagent Co., Ltd. All materials were used directly without purification in advance.
Synthesis of CdS nanowires and CdS/MoS2 nanocomposites The CdS nanowires and CdS/MoS2 nanocomposites were prepared through a modified solvothermal method [32,33] (Scheme 1). Firstly, C4H6CdO4$2H2O (1.1994 g), C8H12Mo2O8 (0.0087 g) and thiourea (1.1276 g) was dispersed in 80 mL C2H8N2 under ultrasonic for 1 h. After that, the mixture solution was transferred into Teflon autoclave (100 mL) and held at 180 C for 72 h. Then, the powders were obtained by using high-speed centrifugation and washed with ethanol and water thoroughly. After that, the powders were dried in vacuum oven at 40 C for 12 h. The weight ratio of MoS2 to CdS was 1:100. The different samples with the weight ratio of MoS2 to CdS were prepared by adding different amount of Mo precursor. On the other hand, the pure CdS nanowires and MoS2 were prepared by similar procedure except that the C8H12Mo2O8 and C4H6CdO4,2H2O were not used, respectively.
Preparation of Pt-CdS and Pt-CdS/MoS2 modified electrodes The CdS and CdS/MoS2 coated on F-doped tin oxide (FTO) electrodes were prepared. Before coating, the FTO electrodes were cleaned by sonication for 10 min under acetone, ethanol, and water, respectively. 10 mg as-synthesized samples were added into 1 mL water-ethanol mixtures (Vwater:Vethanol ¼ 1:1). Then, 10 mL Nafion was added into above dispersion and placed under ultrasound for 30 min to obtain yellow
Scheme 1 e The schematic illustration of the formation process of ultrathin MoS2 sheets enwrapped CdS nanowires and corresponding Pt nanoparticles decorated on CdS/MoS2 composites. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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dispersion. After that, 50 mL of the above dispersion was deposited on the surface of FTO and then dried at 30 C in the vacuum oven, resulting in CdS or CdS/MoS2 modified FTO electrode. The Pt nanoclusters decorated on CdS/FTO and CdS/MoS2/ FTO electrodes were prepared by using the electrodeposition method at 0.2 V in an aqueous solution including of H2PtCl6 (3.0 mM) and KCl (0.5 M), in which Pt wire and saturated calomel electrode (SCE), and modified FTO electrode were acted as the counter and reference electrodes, respectively. Assuming a 100% current efficiency, the loading amounts of Pt on the surface of CdS and CdS/MoS2 were determined by the charge integrated during electrodeposition process. The charge for the deposition of Pt on CdS or CdS/MoS2 was ca. 0.0155 C, which correspond to the Pt loadings of 0.0313 mg. The weight ratio of Pt to CdS or CdS/MoS2 was 0.0626:1.
from adventitious carbon. Photoluminescence (PL) spectra were measured at room temperature on Edinburgh FLS920 fluorospectrophotometer. The excitation wavelength was 405 nm. Hydroxyl radical ($OH) were detected with electron spin resonance (ESR) spectroscopy, which recorded on Bruker EMX ESR Spectrometer (Billerica, MA). 5,5-dimethyl-L-pyrroline-N-oxide (DMPO) was used to spin trap $OH. All the spin trapping ESR measurements for detection of the spin adducts were carried out by using the settings of 10 mW microwave power, 100 G scan range, and 1 G eld modulation.
Electro- and photo-electrochemical measurements
To confirm component and crystallographic phase of the asesynthesized samples, XRD patterns of CdS and CdS/MoS2 were measured (Fig. 1). For pure CdS samples, the peaks at ca. 24.8 , 26.5 , 28.2 , 36.6 , 43.7 , 47.8 , 51.9 and 58.3 could be assigned respectively to the diffraction of (100), (002), (101), (102), (110), (103), (112), and (202) crystal faces of hexagonal wurtzite structure CdS (JCPDS 41-1049) [33]. Comparing to the XRD pattern of the CdS samples, the CdS/MoS2 sample has a main characteristics peaks at ca. 14.2 that can be assigned to the (002) facet of hexagonal phase MoS2 [26]. The peaks at 14.2 and 26.5 are corresponding to the diffraction from (002) plane of MoS2 and CdS with despacing of 0.62 nm and 0.34 nm, respectively. For comparison, the pure 1D CdS nanowires were synthesized by similar process while without Mo source. The morphologies of both samples were analyzed by SEM and TEM, as shown in Fig. 2. It can be seen uniform CdS nanowires with an average diameter of 50 nm and lengths in the range of 2e4 mm (Fig. 2aec). High-resolution TEM (HRTEM, insert of Fig. 2c) of the CdS nanowire exhibits lattice fringes with d spacing of ca. 0.67 nm that can be assigned to the (001) lattice planes of CdS. This also suggests that the growth direction of the CdS nanowire is [001], along the c axis of the nanowires [32].
The electro- and photo-electrochemical measurements were measured in a standard three-electrode cell through a CHI 660B potentiostat/galvanostat (Shanghai Chenhua Instrumental Co., Ltd., China), in which Pt wire, SCE, and modified FTO electrode were acted as the counter, reference, and working electrodes, respectively. The cyclic voltammetries (CVs) of the working electrodes were monitored in the 1.0 M KOH and 1.0 M CH3OH (or CH3CH2OH) solution from 0.9 V to 0.2 V (vs. SCE). The transient photocurrent responses of the modified electrode in 1.0 M CH3OH þ KOH solution were measured with a potential of 0.35 V at a scan rate of 50 mV s1 under visible light illumination. The illumination was interrupted every 30 s. The chronoamperometry curve of the working electrodes under dark and visible light illumination in 1.0 M KOH and 1.0 M CH3OH (or CH3CH2OH) solution were measured for 2000 s at the potential of 0.35 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out by using 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] (1:1) mixture as a redox probe in the KCl (0.1 M) solution. The EIS measurements were recorded with an AC perturbation signal of 5.0 mV over the frequency range from 100 kHz to 0.1 Hz with the potential at 0.25 V (vs.SCE). The CdS or CdS/MoS2 modified electrode was irradiated through a 150 W xenon arc lamp equipped with UV cuteoff filter (>400 nm) for visible light photoelectrochemical experiment.
Results and discussion Characterization of the CdS/MoS2 core/shell nanoarchitecture
Apparatus and measurements Scanning electron microscope (SEM, S-4700, Hitachi) and attached energy dispersive X-ray (EDX) analysis (Horiba EMAX) were used for morphology measurement and energy dispersive spectroscope, respectively. Transmission electron microscopy (TEM) images were obtained from TecnaiG220 instrument. The UVeViseNIR Shimadzu UV3150 spectrophotometer (Japan) was employed for the UVevis diffuse reflectance spectra measurements. X-ray diffraction (XRD) patterns of the samples were recorded on the PANalytical X' Pert PRO MRD system with Cu Ka radiation (k ¼ 1.54056 A) operated at 40 kV and 30 mA. An ESCALab220i-XL electron spectrometer was used for X-ray photoelectron spectroscopy (XPS). The binding energies were referenced to the C 1s line at 284.8 eV
Fig. 1 e XRD patterns of CdS (a) and CdS/MoS2 (b).
Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Fig. 2 e SEM (a, b and d), TEM (c and e) and HRTEM (f) images of CdS nanowires (aec) and CdS/MoS2 core/shell nanoarchitectures (def). The insert of figure c is HRTEM image of CdS nanowire.
Interestingly, in the presence of Mo precursor, 1D CdS nanowires/2D MoS2 nanosheets core/shell heterarchitectures were formed after solvothermal process. The SEM (Fig. 2d) and TEM images (Fig. 2e) show ultrathin MoS2 nanosheets coated CdS nanowire heterostructures. The HRTEM image (Fig. 2f) clearly exhibits core/shell fine structure that CdS nanowire was enwrapped by ultrathin MoS2 nanosheets. The observed lattice fringes with d spacing of ca. 0.34 nm and 0.62 nm are in good agreement with the (002) lattice planes of CdS (JCPDS 411049) [33] and MoS2 [26,27], respectively. To validate the generation of CdS and MoS2 nanostructures, the components of the obtained CdS and CdS/MoS2 heterostructures were investigated by EDX analysis. Fig. S1 A and B shows that the elements S, Cd and S, Cd, Mo were detected in the CdS and CdS/MoS2, respectively, indicating the generation of CdS and CdS/MoS2 nanospecies. Chemical composition of the two samples was further analyzed with XPS, as shown in Fig. 3. Firstly, in the pure CdS nanowires, the peaks attributed to the binding energy for Cd 3d5/2 and Cd 3d3/2 were observed at 405.3 eV and 412.0 eV, respectively (Fig. 3Aa), which is a characteristic of Cd2þ in CdS [27,30]. Moreover, as shown in Fig. 3B-b, two peaks at 162.2 eV and 163.3 eV were assigned to the doublet of S 2p3/2 and S 2p1/2, respectively [26]. On the other hand, in the CdS/MoS2 composites, besides the
Cd 3d and S 2p peaks, a doublet peak centered at 229.1 and 232.4 eV, which agree well with the values of Mo (þ4) 3d5/2 and Mo (þ4) 3d3/2 states (Fig. 3C) [26,27]. The peak at 233.8 eV assigned to Mo (þ6) 3d5/2 was inconspicuous, indicating that the Moþ6 was completely reduced into metallic Moþ4. These results revealed the generation of CdS/MoS2 nanocomposites. Interestingly, the binding energy of S 2p and Cd 3d in the CdS/ MoS2 nanocomposites shift of ~0.2 eV toward lower binding energy. This shift might be due to the interaction of CdS and MoS2 in the composites. Usually, the UVevis diffuse reflectance spectra (UV-DRS) study is used to analyze the absorption properties of the solid samples. As shown in Fig. S2, the UV-DRS of pure CdS, pure MoS2 and CdS/MoS2 were studied. The absorption edge of the CdS was at ca. 537 nm, revealing a narrow band gap of the semiconductor CdS. In the 2D MoS2 sheets enwrapped the CdS fibers, a small shift of absorption edge (542 nm) and a tailing absorption were observed in the visible region. The tailing absorption is owing to the introduction of the MoS2, which has an absorption in visible range (Fig. S2c) [26]. The distinctly absorption in the visible range of the CdS/MoS2 suggests that CdS/MoS2 can be used as an excellent visible light response support for highly efficient photoelectrocatalyst towards MOR under visibleelight illumination.
Fig. 3 e XPS spectra of Cd 3d (A), S 2p (B) of CdS (a) and CdS/MoS2 (b). XPS spectrum of Mo 3d (C) of CdS/MoS2 nanocomposites. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Fig. 4 e A: The CVs of Pt-CdS (a, b), Pt-CdS/MoS2 (c, d), FTO (e) and CdS/MoS2 (f) electrodes in 1.0 M CH3OH and 1.0 M KOH solution at a scan rate of 50 mV s¡1 under dark (a, c, e, f) and visible light illumination (b, d). B: The histogram of activities of different electrodes from the data of Fig. 4A. C: The different weight ratio of CdS/MoS2 for constructing electrodes for electrocatalytic MOR without and with visible light irradiation.
Characterization of the Pt-CdS/MoS2 nanocomposite Pt is known to be the most effective electrocatalyst for MOR in fuel cell. Herein, the CdS nanowires and CdS/MoS2 heterostructures were used as supports for depositing Pt nanoclusters. TEM images of the resulting Pt-deposited samples are shown in Fig. S3. Pt nanocluster with an average size of ca. 4 nm can be seen deposited on both surface of the CdS nanowires and the CdS/MoS2 heterostructures. Furthermore, Fig. S1 C and D shows that the compositions of the Pt-CdS and Pt-CdS/MoS2 nanocomposites were analyzed by EDX experiment. The elements S, Cd, Mo and Pt were detected in asesynthesized samples. XPS was also analyzed to confirm the chemical composition of the Pt-CdS and Pt-CdS/MoS2 nanocomposites, as shown in Fig. S4. It can be seen that the Pt 4f binding energy was observed in both samples. A doublet peak originating from the spin orbital splitting of the Pt 4f5/2 and Pt 4f7/2 states are centered at 75.1 and 71.7 eV, respectively. This result agrees well with the values for metallic Pt [4,34], further confirming the formation of Pt-CdS and Pt-CdS/ MoS2 nanocomposites. The weight and atom contents for different elements in the as-synthesized Pt-CdS/MoS2 were summarized in Table S1, which is in agreement with theoretical predictions.
Photoelectrocatalytic performance The photoelectrocatalytic activity of the as-prepared Pt-CdS/ MoS2 and other comparative compounds modified electrode were evaluated by MOR (Fig. 4, Table 1). Firstly, cyclic
voltammetries (CVs) in Fig. 4A for electrodes in 1.0 M CH3OH and 1.0 M KOH solution with a scan rate of 50 mV s1 either in dark or with visible light illumination were studied. The CVs display a typical profile for the electrocatalytic MOR from 0.9 V to 0.2 V, with strong oxidation peaks at ca. 0.25 V in forward scan and ca. 0.4 V in backward scan. The peak at ca. 0.4 V was related oxidation of CO and other carbonaceous species results from incomplete oxidation of methanol. The pure CdS or MoS2 didn't show any catalytic activity under this condition (result not shown). When Pt NPs were deposited on the surface of CdS, the current density of MOR on the Pt-CdS modified electrode at ca. 0.25 V (vs. SCE) was ca. 0.81 mA cm2. As the CdS possesses nice visible absorption, when the electrode was exposed to visible light, superior performance emerging from the electrocatalytic MOR was observed. The current density of Pt-CdS modified electrode was improved to 2.23 mA cm2 under visible light illumination. When Pt nanoclusters were decorated on the surface of the CdS/MoS2 core/shell heterostructures, the current density of MOR was reached to 1.13 mA cm2. Interestingly, the ternary Pt-CdS/MoS2 modified electrode displayed the highest current density of MOR (4.56 mA cm2) with visible light irradiation. This value is approximately 5.63 and 2.04 times than that of Pt-CdS modified electrode with and without visible light irradiation, and 4.04 times than that of Pt-CdS/ MoS2 modified electrode in dark. Furthermore, dependence of the photoelectrocatalytic activity on the weight ratio of CdS and MoS2 was investigated, as shown in Fig. 4C. The optimal amount of CdS/MoS2 in the present photoelectrocatalytic system was found to be 100:1. A further increase in loading
Table 1 e A summarization of the electrocatalytic performances of as-prepared electrodes toward the oxidation of CH3OH and CH3CH2OH in alkaline medium. Electrode Pt-CdS Pt-CdS/MoS2 a b c d
Current density/mA cm2a,c
Current density/mA cm2a,d
Current density/mA cm2b,c
Current density/mA cm2b,d
0.81 1.13
0.22 0.42
2.23 4.56
0.61 1.32
Without light. Under visible light irradiation. CH3OH oxidation. CH3CH2OH oxidation.
Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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MoS2 may block the active sites on the CdS surface and decrease light absorption on the CdS nanowires, leading to a decrease in activity. Beside methanol oxidation, both our Pt-CdS and Pt-CdS/ MoS2 modified electrodes displayed excellent catalytic performance on ethanol oxidation. As shown in Table 1 and Fig. S5, the current densities of ethanol oxidation of the Pt-CdS and Pt-CdS/MoS2 modified electrodes were ca. 0.22 and 0.42 mA cm2, respectively. Under visible light illumination, the catalytic performances of the two electrodes were distinctly improved. The ternary Pt-CdS/MoS2 modified electrode in visible light irradiation displayed the highest current density of ethanol oxidation (ca. 1.32 mA cm2) compared to Pt-CdS modified electrode with and without visible light irradiation, and Pt-CdS/MoS2 modified electrode in dark.
Stability of Pt-CdS/MoS2 electrodes Longeterm stability is another important factor for evaluating a catalyst in practical application. In order to investigate the long-period of stability of electrodes, the scan cycling experiments and chronoamperometric curves of the two electrodes were studied with and without visible light illumination. From Fig. 5A, the oxidation peak current density of the Pt-CdS modified electrode was lower compared to the Pt-CdS/MoS2 modified electrode after 400 cycles. When the Pt-CdS modified electrode was under visible light illumination, the oxidation peak current density increased greatly at the initial stage. However, the oxidation peak current density decreased by ca. 43% after 400 cycles compared to the maximum value of 2.23 mA cm2 at the 100th. When Pt-CdS/MoS2 was used as electrode, the oxidation peak current densities displayed the highest and arrived to the maximum value of 4.56 mA cm2 at the 100th under visible light irradiation. Moreover, the oxidation peak current density of Pt-CdS/MoS2 modified electrode only decreased by ca. 21% after 400 cycles. Fig. 5B shows the chronoamperometric curves of MOR on above electrodes under dark and visible-light illumination. It also displayed that the Pt-CdS/MoS2 modified electrode shows the highest initial and steady-state of oxidation peak current densities under light irradiation, which suggest that the photo-irradiation was beneficial for the electrocatalytic activity and stability. Similar phenomenon was also observed on the as-prepared electrodes for oxidation of ethanol, as shown in Figs. S6 and 7.
Photoluminescence (PL) and photocurrent property The above results show that compared to bare Pt-CdS, the introduced 2D MoS2 nanosheets displayed apparently enhanced catalytic performance both under dark and light illumination environment. There are several reasons contribute this enhancement. Firstly, 2D MoS2 nanosheets exhibit a relative large surface area, which plays an important role in the improvement of catalytic activity. Moreover, an efficient charge separation is crucial during photoelectrocatalytic process. Herein, pen-junction nanohybrid by integrating with p-type of MoS2 and n-type CdS semiconductor, an improved separation of photoinduced electrons and holes can be easily reached. To investigate this interfacial charge transfer between CdS and MoS2 in heterostructures, we first performed photoluminescence (PL) emission and time-resolved fluorescence decays of pure CdS and CdS/MoS2 heterostructures. Fig. 6A shows typical PL emission spectrum of CdS, which at ca. 507 nm (2.44 eV, with respect to the bandgap of CdS) was observed when CdS nanowires were excited at 405 nm. However, when CdS was enwrapped with MoS2 nanosheets, substantial PL emission quenching with ca. 86% were clearly observed. This result indicates that there is much less electronehole recombination and the electron from excited CdS can be efficiently trapped by MoS2 [35,36]. The kinetics of an electron transfer from excited CdS to MoS2 was further probed by the time-resolved emission decay experiment. The corresponding fitted fluorescence decay data of Fig. 6B are listed in Table 2. It can be seen that the fluorescence lifetimes (t) of CdS and CdS/MoS2 are 1.17 and 0.41 ns. It's well known that the sensitizer bound on a quenching material had a shorter fluorescence lifetime than the free sensitizer. This phenomenon could also be correlated with the interfacial electron-transfer process [35]. The electrontransfer rate constant (kET) are listed in Table 2. The kET was ca. 1.58 109 s between CdS and CdS/MoS2. The rapid electron transfer rate is attributed to the fact that the ultrathin MoS2 sheets enwrapped CdS directly, resulting in a greatly suppressed electronehole recombination in CdS/MoS2 nanocomposites under light irradiation. The responsive photocurrents were also carried out to evaluate the photoelectrocatalytic performance of the CdS and CdS/MoS2 electrode under visible light irradiation. As shown in Fig. 6C, during the light “on” and “off” conditions at
Fig. 5 e The peak current of methanol oxidation in the forward scan vs. the CV cycle number (A) and chronoamperometric curves (B) on Pt-CdS (a, b) and Pt-CdS/MoS2 (c, d) electrode under dark (a, c) and visible light illumination (b, d). Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Fig. 6 e PL mission spectra (A), decay profiles (B) and photocurrent responses (C) observed for CdS (a) and CdS/MoS2 (b). Excitation wavelength: 405 nm. The photocurrent responses of the working electrode was under visible light irradiation in 1.0 M KOH and 1.0 M CH3OH solution at a potential of ¡0.35 V with a scan rate of 50 mV s¡1.
the electrode, the rise and fall off the current are very clear. It is found that when the CdS modified electrode was under visible light irradiation, there was only a responsive photocurrents with ca. 0.05 mA cm2 generated, while the CdS/MoS2 displayed 0.27 mA cm2. The MoS2-based electrode was approximately 5.5 times higher than that of the CdS modified electrode, suggesting effective charger transfer in the CdS/ MoS2 heterostructure.
CVs and electrochemical impedance spectroscopy (EIS) property To further evaluate the improved charge transfer efficiencies during the photo-reaction, CVs and electrochemical impedance spectroscopy (EIS) of Pt-CdS and Pt-CdS/MoS2 electrode in 0.1 M KCl and 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] solution with a potential of 0.25 V under dark and visible light illumination were carried out. Firstly, CVs (Fig. 7A) show a distinctly ). It can be seen that reversible redox couple (i.e., Fe(CN)4/3 6 the redox peak current of Pt-CdS and Pt-CdS/MoS2 distinctly enhanced with assisted visible light irradiation compared to
Table 2 e Time-resolved fluorescence decay data of CdS and CdS/MoS2 derived from Fig. 6B. Sample CdS CdS/MoS2 a
t1 (ns)
t2 (ns)
tave (ns)
kET (s1)a
5.45 (11%) 4.72 (4%)
0.64 (89%) 0.24 (96%)
1.17 0.41
e 1.58 109
Electron-transfer rate constants (kET) is calculated according to ðtCdS=MoS2 Þ1 (tCdS)1.
that of without light illuminated. This result suggests an improvement of interfacial electron transfer rate between redox couples in the solution and electrode surface under light irradiation [4,37]. Moreover, to investigate the internal resistance and interfacial charge transfer of the electrodes under dark and visible light irradiation, EIS data were analyzed, as shown in Fig. 7B. As shown, the radius of semicircle arc of Pt-CdS/MoS2 electrode was much smaller than that of Pt-CdS modified electrode under similar condition. Furthermore, when Pt-CdS/MoS2 modified electrode was under visible light illumination, the radius of semicircle arc was much smaller than that of same electrode under dark and Pt-CdS electrode under visible light illumination, suggesting a decrease in the solid state interface layer resistance and an increase of charge transfer on the surface [38,39]. The corresponding equivalent circuit was used to fit the EIS datum of Pt-CdS/MoS2 under light irradiation, shown in Fig. 7C. Where Rs represents the electrolyte resistance, Q is the electrode double-layer capacitance formed at PtCdS/MoS2/solution interface, Rct is the charge-transfer resistance at Pt-CdS/MoS2/solution interface, Ro is the chargetransfer resistance at FTO/solution interface, S is Warburg diffusion impedance and Co represents the capacitance of the composite materials on the surface of the electrode. Beside the K3 [Fe(CN)6/K4 [Fe(CN)6] as redox probe, the corresponding EIS spectra of different conditions were also measured in 1.0 M CH3OH and 1.0 M KOH solution, similar results were observed (Fig. S8). The parameters of Rct for both electrolytes were summarized in Table 3. It's more easily to see that the Pt-CdS/ MoS2 displayed smallest resistance under visible light irradiation, which is in accordance with the EIS spectra.
Fig. 7 e CVs (A) and (B) EIS spectra of Pt-CdS (a, b) and Pt-CdS/MoS2 (c, d) electrode in of in 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] and 0.1 M KCl solution at a potential of 0.25 V under dark (a, c) and visible light irradiation (b, d). C: Equivalent circuit was used for simulating the impedance spectrum on the Pt-CdS/MoS2 electrode under visible light irradiation. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Table 3 e EIS fitting parameters of Rct from equivalent circuits for different samples from Fig. 7C and Fig. S8B. Electrode Rct from Fig. 7C Rct from Fig. S8B a b
Pt-CdSa
Pt-CdSb 2
680.0 U cm 7647 U cm2
Pt-CdS/MoS2a 2
290.1 U cm 2747 U cm2
2
312.4 U cm 3225.0 U cm2
Pt-CdS/MoS2b 223.7 U cm2 1530 U cm2
Without light. Under visible light irradiation.
Based on above results, CdS/MoS2 heterostructures acted as efficient photo-activated support for depositing Pt nanoclusters. Great improvements of activity and stability for MOR under visible light irradiation were achieved with this system. This is due to the synergistic effects of photocatalytic and electrocatalytic processes during catalytic methanol oxidation, as show in Scheme 2. Firstly, the methanol can be oxidized on the surface of Pt nanoclusters by traditional electrocatalytic process. At the same time, when the electrode is under visible light illumination, the CdS nanowires absorb light energy, resulting in electrons and holes. The interfacial electron transfer from the excited CdS moieties to the decorated ultrathin MoS2 shell contributes to efficient charger separation (demonstrated by Figs. 6 and 7). The holes are positive enough to oxidized OH/H2O to hydroxyl radicals ($OHs) at the surface of electrode [4e8]. These $OHs are strong oxidant species for oxidizing methanol, resulting in photooxidation of methanol at the surface of electrode [4e8]. Furthermore, the reactive radical species will oxidize the intermediate carbonaceous species like COads, which lead to an efficiently poisoning suppression [10,12]. Therein, the catalytic activity and stability of MOR on as-prepared electrode can be improved efficiently with visible light assisted. To demonstrate the generation of the $OHs in the CdS/ MoS2 during visible light irradiation, electron spin resonance (ESR) spectrum with DMPO spin trapping adducts were thus used to check the generation of $OH in the CdS/MoS2/air system. As shown in Fig. 8, four-line spectra with relative
Fig. 8 e DMPO spin-trapping electron spin resonance spectra of CdS/MoS2 nanocomposite for DMPO-OH.
intensities of 1:2:2:1 were observed in the aqueous dispersions of CdS/MoS2. This result clearly shows the generation of $OH in the presence of CdS/MoS2 under visible light irradiation [40]. These $OHs will synergistic improve the catalytic oxidization of methanol during photoelectrocatalytic process (Scheme 2).
Scheme 2 e Proposed schematic illustration for synergistic photoe and electroe catalytic MOR process by using Pt-CdS/ MoS2 core/shell nanoarchitectures modified electrode under visibleelight illumination.
Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Conclusions [7]
In summary, ultrathin 2D MoS2 sheets coated 1D CdS nanowires core/shell heterostructures were synthesized by using one-step solvothermal method. After depositing Pt nanoclusters, high performances visible-light enhanced electrocatalytic oxidation of alcohol were clearly observed comparing with traditional ambient electrocatalytic oxidation. The efficient interfacial electron transfer from the excited CdS nanowires to ultrathin MoS2 shell helps to improve the synergistic effect of photocatalytic and electrocatalytic process for boosting of catalytic alcohol oxidation. The outstanding catalytic performance shows that the CdS nanowire/MoS2 nanosheet core/shell heterarchitecture could act as a promising photo-activated material in the fields of solar and chemical energy conversion and this result also provides new insight into the development of visible light photo-activated electrode for applications in fuel cells.
[8]
[9]
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Acknowledgment This work was sponsored by K.C. Wang Magna Fund in Ningbo University. The authors also appreciate to the NSFC (21603111, 51373111), Suzhou Nanoeproject (ZXG2012022), and the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207).
[13]
[14]
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Appendix A. Supplementary data [16]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.035. [17]
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Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires Chunyang Zhai a, Mingjuan Sun a, Mingshan Zhu a,*, Ke Zhang b, Yukou Du b a b
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, PR China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
In this paper, a new and high-performance visible-light-activated electrode is designed in
Received 26 August 2016
terms of two dimensional (2D) MoS2 nanosheets enwrapped 1D CdS nanowires as photo-
Received in revised form
activated support for depositing Pt nanoclusters. Compare to traditional ambient electro-
29 October 2016
catalytic process, the as-prepared photo-activated electrode shows evidently improved
Accepted 4 November 2016
electrocatalytic activity and stability of methanol oxidation reaction (MOR) under visible
Available online xxx
light illumination. The efficient interfacial electron transfer from the excited CdS moieties
Keywords:
and electrocatalytic process for the boosting of catalytic efficiency.
to the decorated ultrathin MoS2 shell contributes to the synergistic effect of photocatalytic Methanol oxidation
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Molybdenum disulfide Cadmium sulfide Visible light
Introduction Fuel cell, as a technology for direct conversion of the chemical energy of a fuel into electrical energy, has been recognized as a promising future power source [1]. Recently, traditional noble metal anode electrocatalysts hybridized with optically active semiconductors have been certified as the promising photoelectrocatalysts for the improvement of fuel cell anodic reactions [2e12]. With assistance of light illumination, those of noble metal/semiconductor modified electrodes displayed evidently boosted catalytic performance and stability for the
anodic oxidation of small organic molecules (SOMs). Despite some efforts have been devoted to the study of photoassisted fuel cell, most of optically active semiconductors have localized in UV-light-activated TiO2, which might hinder their broad applications. Multifarious nanostructures with different dimensions including zero-, one-, two-, and three-dimensional (0D, 1D, 2D and 3D), have received wide recognition for their unique sizee and shapeedependent physicochemical properties [13e16]. Among these nanostructures, well-defined 1D nanoarchitectures such as wires, rods, belts, etc. have become the focus of intensive research owing to their unique large aspect
* Corresponding author. E-mail address: [email protected] (M. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2016.11.035 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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ratio, which is beneficial to a directional charge transfer with a reduced grain boundary scattering, since the discovery of carbon nanotubes by Iijima [14e20]. Nowadays, the 1D nanomaterials play important roles as key units into nanoscale optic, electronic, electrochemical and particular interest to photo-/electro-catalytic fields [14,15,19,20]. 1D CdS nanomaterials are fascinating and promising optoelectronical material for the energy conversion [19,20]. This is because of its unique and versatile fundamental properties including desired band-gap width (2.4 eV), relative low work function, high refraction index, excellent transport properties, high electronic mobility, etc. However, the pure CdS suffers from the rapid recombination of photogenerated electrons and holes, and also severe photocorrosion effect. Recently, molybdenum sulfide (MoS2) nanosheet, a 2D chalcogenide nanomaterial with the same hexagonal crystalline structure as CdS nanoparticles (NPs), is often used to improve the photocatalytic performance of CdS nanostructures [21e31]. Exhibiting p-type properties, the 2D MoS2 nanosheets with large surface area can be integrated with an n-type CdS semiconductor to improve the separation of photoinduced electrons and holes. For example, Li's group had reported that MoS2 as a cocatalyst loaded on CdS NPs for the enhancement of photocatalytic H2 evolution under visible light irradiation [22,23]. Zhang et al. synthesized 2D MoS2/CdS pen nanohybrids through a one-pot solvothermal process for photocatalytic H2 production [25]. However, the above investigations limited CdS NPs modified MoS2 sheets, we expect that it is necessary to construct a core/shell nanoarchitecture with 1D CdS as core and 2D MoS2 sheets as shell for the investigation of charge separation and interfacial electron transfer of CdS/MoS2. Moreover, compared to massive studies of high-efficiency photocatalytic H2 evolution on the CdS/MoS2 heterostructure, comparatively few reports have been published on this hybrid for other applications. Herein, we reported the fabrication of a novel and highperformance visible-light-activated electrode in terms of 2D MoS2 nanosheets enwrapped 1D CdS nanowires. Firstly, by means of a one-pot solvothermal method, core/shell heterostructures with ultrathin 2D MoS2 sheets coated 1D CdS nanowires were easily obtained (Scheme 1). Then, the asprepared heterostructures can be served as support for depositing Pt nanoclusters. Interestingly, compared to traditional methanol oxidation reaction (MOR), the ternary as-prepared Pt-CdS/MoS2 photoanode showed evidently improved electrocatalytic activity and stability of MOR under visible-light illumination. The highly efficient interfacial
charger transfer from excited CdS moieties to decorated ultrathin MoS2 shell contributes to the synergistic effect of photocatalytic and electrocatalytic process for the boosting of catalytic methanol oxidation. The outstanding catalytic performance suggests that the CdS nanowire/MoS2 nanosheet core/shell heterarchitecture could act as a promising photoactivated material in solar and chemical energy conversion.
Experimental Materials Thiourea (CH4N2S), cadmium acetate dihydrate (C4H6CdO4$2H2O), molybdenum (II) acetate dimer (C8H12Mo2O8), H2PtCl6$6H2O, and ethylenediamine (C2H8N2) were purchased from Sinopharm Chemical Reagent Co., Ltd. All materials were used directly without purification in advance.
Synthesis of CdS nanowires and CdS/MoS2 nanocomposites The CdS nanowires and CdS/MoS2 nanocomposites were prepared through a modified solvothermal method [32,33] (Scheme 1). Firstly, C4H6CdO4$2H2O (1.1994 g), C8H12Mo2O8 (0.0087 g) and thiourea (1.1276 g) was dispersed in 80 mL C2H8N2 under ultrasonic for 1 h. After that, the mixture solution was transferred into Teflon autoclave (100 mL) and held at 180 C for 72 h. Then, the powders were obtained by using high-speed centrifugation and washed with ethanol and water thoroughly. After that, the powders were dried in vacuum oven at 40 C for 12 h. The weight ratio of MoS2 to CdS was 1:100. The different samples with the weight ratio of MoS2 to CdS were prepared by adding different amount of Mo precursor. On the other hand, the pure CdS nanowires and MoS2 were prepared by similar procedure except that the C8H12Mo2O8 and C4H6CdO4,2H2O were not used, respectively.
Preparation of Pt-CdS and Pt-CdS/MoS2 modified electrodes The CdS and CdS/MoS2 coated on F-doped tin oxide (FTO) electrodes were prepared. Before coating, the FTO electrodes were cleaned by sonication for 10 min under acetone, ethanol, and water, respectively. 10 mg as-synthesized samples were added into 1 mL water-ethanol mixtures (Vwater:Vethanol ¼ 1:1). Then, 10 mL Nafion was added into above dispersion and placed under ultrasound for 30 min to obtain yellow
Scheme 1 e The schematic illustration of the formation process of ultrathin MoS2 sheets enwrapped CdS nanowires and corresponding Pt nanoparticles decorated on CdS/MoS2 composites. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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dispersion. After that, 50 mL of the above dispersion was deposited on the surface of FTO and then dried at 30 C in the vacuum oven, resulting in CdS or CdS/MoS2 modified FTO electrode. The Pt nanoclusters decorated on CdS/FTO and CdS/MoS2/ FTO electrodes were prepared by using the electrodeposition method at 0.2 V in an aqueous solution including of H2PtCl6 (3.0 mM) and KCl (0.5 M), in which Pt wire and saturated calomel electrode (SCE), and modified FTO electrode were acted as the counter and reference electrodes, respectively. Assuming a 100% current efficiency, the loading amounts of Pt on the surface of CdS and CdS/MoS2 were determined by the charge integrated during electrodeposition process. The charge for the deposition of Pt on CdS or CdS/MoS2 was ca. 0.0155 C, which correspond to the Pt loadings of 0.0313 mg. The weight ratio of Pt to CdS or CdS/MoS2 was 0.0626:1.
from adventitious carbon. Photoluminescence (PL) spectra were measured at room temperature on Edinburgh FLS920 fluorospectrophotometer. The excitation wavelength was 405 nm. Hydroxyl radical ($OH) were detected with electron spin resonance (ESR) spectroscopy, which recorded on Bruker EMX ESR Spectrometer (Billerica, MA). 5,5-dimethyl-L-pyrroline-N-oxide (DMPO) was used to spin trap $OH. All the spin trapping ESR measurements for detection of the spin adducts were carried out by using the settings of 10 mW microwave power, 100 G scan range, and 1 G eld modulation.
Electro- and photo-electrochemical measurements
To confirm component and crystallographic phase of the asesynthesized samples, XRD patterns of CdS and CdS/MoS2 were measured (Fig. 1). For pure CdS samples, the peaks at ca. 24.8 , 26.5 , 28.2 , 36.6 , 43.7 , 47.8 , 51.9 and 58.3 could be assigned respectively to the diffraction of (100), (002), (101), (102), (110), (103), (112), and (202) crystal faces of hexagonal wurtzite structure CdS (JCPDS 41-1049) [33]. Comparing to the XRD pattern of the CdS samples, the CdS/MoS2 sample has a main characteristics peaks at ca. 14.2 that can be assigned to the (002) facet of hexagonal phase MoS2 [26]. The peaks at 14.2 and 26.5 are corresponding to the diffraction from (002) plane of MoS2 and CdS with despacing of 0.62 nm and 0.34 nm, respectively. For comparison, the pure 1D CdS nanowires were synthesized by similar process while without Mo source. The morphologies of both samples were analyzed by SEM and TEM, as shown in Fig. 2. It can be seen uniform CdS nanowires with an average diameter of 50 nm and lengths in the range of 2e4 mm (Fig. 2aec). High-resolution TEM (HRTEM, insert of Fig. 2c) of the CdS nanowire exhibits lattice fringes with d spacing of ca. 0.67 nm that can be assigned to the (001) lattice planes of CdS. This also suggests that the growth direction of the CdS nanowire is [001], along the c axis of the nanowires [32].
The electro- and photo-electrochemical measurements were measured in a standard three-electrode cell through a CHI 660B potentiostat/galvanostat (Shanghai Chenhua Instrumental Co., Ltd., China), in which Pt wire, SCE, and modified FTO electrode were acted as the counter, reference, and working electrodes, respectively. The cyclic voltammetries (CVs) of the working electrodes were monitored in the 1.0 M KOH and 1.0 M CH3OH (or CH3CH2OH) solution from 0.9 V to 0.2 V (vs. SCE). The transient photocurrent responses of the modified electrode in 1.0 M CH3OH þ KOH solution were measured with a potential of 0.35 V at a scan rate of 50 mV s1 under visible light illumination. The illumination was interrupted every 30 s. The chronoamperometry curve of the working electrodes under dark and visible light illumination in 1.0 M KOH and 1.0 M CH3OH (or CH3CH2OH) solution were measured for 2000 s at the potential of 0.35 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out by using 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] (1:1) mixture as a redox probe in the KCl (0.1 M) solution. The EIS measurements were recorded with an AC perturbation signal of 5.0 mV over the frequency range from 100 kHz to 0.1 Hz with the potential at 0.25 V (vs.SCE). The CdS or CdS/MoS2 modified electrode was irradiated through a 150 W xenon arc lamp equipped with UV cuteoff filter (>400 nm) for visible light photoelectrochemical experiment.
Results and discussion Characterization of the CdS/MoS2 core/shell nanoarchitecture
Apparatus and measurements Scanning electron microscope (SEM, S-4700, Hitachi) and attached energy dispersive X-ray (EDX) analysis (Horiba EMAX) were used for morphology measurement and energy dispersive spectroscope, respectively. Transmission electron microscopy (TEM) images were obtained from TecnaiG220 instrument. The UVeViseNIR Shimadzu UV3150 spectrophotometer (Japan) was employed for the UVevis diffuse reflectance spectra measurements. X-ray diffraction (XRD) patterns of the samples were recorded on the PANalytical X' Pert PRO MRD system with Cu Ka radiation (k ¼ 1.54056 A) operated at 40 kV and 30 mA. An ESCALab220i-XL electron spectrometer was used for X-ray photoelectron spectroscopy (XPS). The binding energies were referenced to the C 1s line at 284.8 eV
Fig. 1 e XRD patterns of CdS (a) and CdS/MoS2 (b).
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Fig. 2 e SEM (a, b and d), TEM (c and e) and HRTEM (f) images of CdS nanowires (aec) and CdS/MoS2 core/shell nanoarchitectures (def). The insert of figure c is HRTEM image of CdS nanowire.
Interestingly, in the presence of Mo precursor, 1D CdS nanowires/2D MoS2 nanosheets core/shell heterarchitectures were formed after solvothermal process. The SEM (Fig. 2d) and TEM images (Fig. 2e) show ultrathin MoS2 nanosheets coated CdS nanowire heterostructures. The HRTEM image (Fig. 2f) clearly exhibits core/shell fine structure that CdS nanowire was enwrapped by ultrathin MoS2 nanosheets. The observed lattice fringes with d spacing of ca. 0.34 nm and 0.62 nm are in good agreement with the (002) lattice planes of CdS (JCPDS 411049) [33] and MoS2 [26,27], respectively. To validate the generation of CdS and MoS2 nanostructures, the components of the obtained CdS and CdS/MoS2 heterostructures were investigated by EDX analysis. Fig. S1 A and B shows that the elements S, Cd and S, Cd, Mo were detected in the CdS and CdS/MoS2, respectively, indicating the generation of CdS and CdS/MoS2 nanospecies. Chemical composition of the two samples was further analyzed with XPS, as shown in Fig. 3. Firstly, in the pure CdS nanowires, the peaks attributed to the binding energy for Cd 3d5/2 and Cd 3d3/2 were observed at 405.3 eV and 412.0 eV, respectively (Fig. 3Aa), which is a characteristic of Cd2þ in CdS [27,30]. Moreover, as shown in Fig. 3B-b, two peaks at 162.2 eV and 163.3 eV were assigned to the doublet of S 2p3/2 and S 2p1/2, respectively [26]. On the other hand, in the CdS/MoS2 composites, besides the
Cd 3d and S 2p peaks, a doublet peak centered at 229.1 and 232.4 eV, which agree well with the values of Mo (þ4) 3d5/2 and Mo (þ4) 3d3/2 states (Fig. 3C) [26,27]. The peak at 233.8 eV assigned to Mo (þ6) 3d5/2 was inconspicuous, indicating that the Moþ6 was completely reduced into metallic Moþ4. These results revealed the generation of CdS/MoS2 nanocomposites. Interestingly, the binding energy of S 2p and Cd 3d in the CdS/ MoS2 nanocomposites shift of ~0.2 eV toward lower binding energy. This shift might be due to the interaction of CdS and MoS2 in the composites. Usually, the UVevis diffuse reflectance spectra (UV-DRS) study is used to analyze the absorption properties of the solid samples. As shown in Fig. S2, the UV-DRS of pure CdS, pure MoS2 and CdS/MoS2 were studied. The absorption edge of the CdS was at ca. 537 nm, revealing a narrow band gap of the semiconductor CdS. In the 2D MoS2 sheets enwrapped the CdS fibers, a small shift of absorption edge (542 nm) and a tailing absorption were observed in the visible region. The tailing absorption is owing to the introduction of the MoS2, which has an absorption in visible range (Fig. S2c) [26]. The distinctly absorption in the visible range of the CdS/MoS2 suggests that CdS/MoS2 can be used as an excellent visible light response support for highly efficient photoelectrocatalyst towards MOR under visibleelight illumination.
Fig. 3 e XPS spectra of Cd 3d (A), S 2p (B) of CdS (a) and CdS/MoS2 (b). XPS spectrum of Mo 3d (C) of CdS/MoS2 nanocomposites. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Fig. 4 e A: The CVs of Pt-CdS (a, b), Pt-CdS/MoS2 (c, d), FTO (e) and CdS/MoS2 (f) electrodes in 1.0 M CH3OH and 1.0 M KOH solution at a scan rate of 50 mV s¡1 under dark (a, c, e, f) and visible light illumination (b, d). B: The histogram of activities of different electrodes from the data of Fig. 4A. C: The different weight ratio of CdS/MoS2 for constructing electrodes for electrocatalytic MOR without and with visible light irradiation.
Characterization of the Pt-CdS/MoS2 nanocomposite Pt is known to be the most effective electrocatalyst for MOR in fuel cell. Herein, the CdS nanowires and CdS/MoS2 heterostructures were used as supports for depositing Pt nanoclusters. TEM images of the resulting Pt-deposited samples are shown in Fig. S3. Pt nanocluster with an average size of ca. 4 nm can be seen deposited on both surface of the CdS nanowires and the CdS/MoS2 heterostructures. Furthermore, Fig. S1 C and D shows that the compositions of the Pt-CdS and Pt-CdS/MoS2 nanocomposites were analyzed by EDX experiment. The elements S, Cd, Mo and Pt were detected in asesynthesized samples. XPS was also analyzed to confirm the chemical composition of the Pt-CdS and Pt-CdS/MoS2 nanocomposites, as shown in Fig. S4. It can be seen that the Pt 4f binding energy was observed in both samples. A doublet peak originating from the spin orbital splitting of the Pt 4f5/2 and Pt 4f7/2 states are centered at 75.1 and 71.7 eV, respectively. This result agrees well with the values for metallic Pt [4,34], further confirming the formation of Pt-CdS and Pt-CdS/ MoS2 nanocomposites. The weight and atom contents for different elements in the as-synthesized Pt-CdS/MoS2 were summarized in Table S1, which is in agreement with theoretical predictions.
Photoelectrocatalytic performance The photoelectrocatalytic activity of the as-prepared Pt-CdS/ MoS2 and other comparative compounds modified electrode were evaluated by MOR (Fig. 4, Table 1). Firstly, cyclic
voltammetries (CVs) in Fig. 4A for electrodes in 1.0 M CH3OH and 1.0 M KOH solution with a scan rate of 50 mV s1 either in dark or with visible light illumination were studied. The CVs display a typical profile for the electrocatalytic MOR from 0.9 V to 0.2 V, with strong oxidation peaks at ca. 0.25 V in forward scan and ca. 0.4 V in backward scan. The peak at ca. 0.4 V was related oxidation of CO and other carbonaceous species results from incomplete oxidation of methanol. The pure CdS or MoS2 didn't show any catalytic activity under this condition (result not shown). When Pt NPs were deposited on the surface of CdS, the current density of MOR on the Pt-CdS modified electrode at ca. 0.25 V (vs. SCE) was ca. 0.81 mA cm2. As the CdS possesses nice visible absorption, when the electrode was exposed to visible light, superior performance emerging from the electrocatalytic MOR was observed. The current density of Pt-CdS modified electrode was improved to 2.23 mA cm2 under visible light illumination. When Pt nanoclusters were decorated on the surface of the CdS/MoS2 core/shell heterostructures, the current density of MOR was reached to 1.13 mA cm2. Interestingly, the ternary Pt-CdS/MoS2 modified electrode displayed the highest current density of MOR (4.56 mA cm2) with visible light irradiation. This value is approximately 5.63 and 2.04 times than that of Pt-CdS modified electrode with and without visible light irradiation, and 4.04 times than that of Pt-CdS/ MoS2 modified electrode in dark. Furthermore, dependence of the photoelectrocatalytic activity on the weight ratio of CdS and MoS2 was investigated, as shown in Fig. 4C. The optimal amount of CdS/MoS2 in the present photoelectrocatalytic system was found to be 100:1. A further increase in loading
Table 1 e A summarization of the electrocatalytic performances of as-prepared electrodes toward the oxidation of CH3OH and CH3CH2OH in alkaline medium. Electrode Pt-CdS Pt-CdS/MoS2 a b c d
Current density/mA cm2a,c
Current density/mA cm2a,d
Current density/mA cm2b,c
Current density/mA cm2b,d
0.81 1.13
0.22 0.42
2.23 4.56
0.61 1.32
Without light. Under visible light irradiation. CH3OH oxidation. CH3CH2OH oxidation.
Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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MoS2 may block the active sites on the CdS surface and decrease light absorption on the CdS nanowires, leading to a decrease in activity. Beside methanol oxidation, both our Pt-CdS and Pt-CdS/ MoS2 modified electrodes displayed excellent catalytic performance on ethanol oxidation. As shown in Table 1 and Fig. S5, the current densities of ethanol oxidation of the Pt-CdS and Pt-CdS/MoS2 modified electrodes were ca. 0.22 and 0.42 mA cm2, respectively. Under visible light illumination, the catalytic performances of the two electrodes were distinctly improved. The ternary Pt-CdS/MoS2 modified electrode in visible light irradiation displayed the highest current density of ethanol oxidation (ca. 1.32 mA cm2) compared to Pt-CdS modified electrode with and without visible light irradiation, and Pt-CdS/MoS2 modified electrode in dark.
Stability of Pt-CdS/MoS2 electrodes Longeterm stability is another important factor for evaluating a catalyst in practical application. In order to investigate the long-period of stability of electrodes, the scan cycling experiments and chronoamperometric curves of the two electrodes were studied with and without visible light illumination. From Fig. 5A, the oxidation peak current density of the Pt-CdS modified electrode was lower compared to the Pt-CdS/MoS2 modified electrode after 400 cycles. When the Pt-CdS modified electrode was under visible light illumination, the oxidation peak current density increased greatly at the initial stage. However, the oxidation peak current density decreased by ca. 43% after 400 cycles compared to the maximum value of 2.23 mA cm2 at the 100th. When Pt-CdS/MoS2 was used as electrode, the oxidation peak current densities displayed the highest and arrived to the maximum value of 4.56 mA cm2 at the 100th under visible light irradiation. Moreover, the oxidation peak current density of Pt-CdS/MoS2 modified electrode only decreased by ca. 21% after 400 cycles. Fig. 5B shows the chronoamperometric curves of MOR on above electrodes under dark and visible-light illumination. It also displayed that the Pt-CdS/MoS2 modified electrode shows the highest initial and steady-state of oxidation peak current densities under light irradiation, which suggest that the photo-irradiation was beneficial for the electrocatalytic activity and stability. Similar phenomenon was also observed on the as-prepared electrodes for oxidation of ethanol, as shown in Figs. S6 and 7.
Photoluminescence (PL) and photocurrent property The above results show that compared to bare Pt-CdS, the introduced 2D MoS2 nanosheets displayed apparently enhanced catalytic performance both under dark and light illumination environment. There are several reasons contribute this enhancement. Firstly, 2D MoS2 nanosheets exhibit a relative large surface area, which plays an important role in the improvement of catalytic activity. Moreover, an efficient charge separation is crucial during photoelectrocatalytic process. Herein, pen-junction nanohybrid by integrating with p-type of MoS2 and n-type CdS semiconductor, an improved separation of photoinduced electrons and holes can be easily reached. To investigate this interfacial charge transfer between CdS and MoS2 in heterostructures, we first performed photoluminescence (PL) emission and time-resolved fluorescence decays of pure CdS and CdS/MoS2 heterostructures. Fig. 6A shows typical PL emission spectrum of CdS, which at ca. 507 nm (2.44 eV, with respect to the bandgap of CdS) was observed when CdS nanowires were excited at 405 nm. However, when CdS was enwrapped with MoS2 nanosheets, substantial PL emission quenching with ca. 86% were clearly observed. This result indicates that there is much less electronehole recombination and the electron from excited CdS can be efficiently trapped by MoS2 [35,36]. The kinetics of an electron transfer from excited CdS to MoS2 was further probed by the time-resolved emission decay experiment. The corresponding fitted fluorescence decay data of Fig. 6B are listed in Table 2. It can be seen that the fluorescence lifetimes (t) of CdS and CdS/MoS2 are 1.17 and 0.41 ns. It's well known that the sensitizer bound on a quenching material had a shorter fluorescence lifetime than the free sensitizer. This phenomenon could also be correlated with the interfacial electron-transfer process [35]. The electrontransfer rate constant (kET) are listed in Table 2. The kET was ca. 1.58 109 s between CdS and CdS/MoS2. The rapid electron transfer rate is attributed to the fact that the ultrathin MoS2 sheets enwrapped CdS directly, resulting in a greatly suppressed electronehole recombination in CdS/MoS2 nanocomposites under light irradiation. The responsive photocurrents were also carried out to evaluate the photoelectrocatalytic performance of the CdS and CdS/MoS2 electrode under visible light irradiation. As shown in Fig. 6C, during the light “on” and “off” conditions at
Fig. 5 e The peak current of methanol oxidation in the forward scan vs. the CV cycle number (A) and chronoamperometric curves (B) on Pt-CdS (a, b) and Pt-CdS/MoS2 (c, d) electrode under dark (a, c) and visible light illumination (b, d). Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Fig. 6 e PL mission spectra (A), decay profiles (B) and photocurrent responses (C) observed for CdS (a) and CdS/MoS2 (b). Excitation wavelength: 405 nm. The photocurrent responses of the working electrode was under visible light irradiation in 1.0 M KOH and 1.0 M CH3OH solution at a potential of ¡0.35 V with a scan rate of 50 mV s¡1.
the electrode, the rise and fall off the current are very clear. It is found that when the CdS modified electrode was under visible light irradiation, there was only a responsive photocurrents with ca. 0.05 mA cm2 generated, while the CdS/MoS2 displayed 0.27 mA cm2. The MoS2-based electrode was approximately 5.5 times higher than that of the CdS modified electrode, suggesting effective charger transfer in the CdS/ MoS2 heterostructure.
CVs and electrochemical impedance spectroscopy (EIS) property To further evaluate the improved charge transfer efficiencies during the photo-reaction, CVs and electrochemical impedance spectroscopy (EIS) of Pt-CdS and Pt-CdS/MoS2 electrode in 0.1 M KCl and 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] solution with a potential of 0.25 V under dark and visible light illumination were carried out. Firstly, CVs (Fig. 7A) show a distinctly ). It can be seen that reversible redox couple (i.e., Fe(CN)4/3 6 the redox peak current of Pt-CdS and Pt-CdS/MoS2 distinctly enhanced with assisted visible light irradiation compared to
Table 2 e Time-resolved fluorescence decay data of CdS and CdS/MoS2 derived from Fig. 6B. Sample CdS CdS/MoS2 a
t1 (ns)
t2 (ns)
tave (ns)
kET (s1)a
5.45 (11%) 4.72 (4%)
0.64 (89%) 0.24 (96%)
1.17 0.41
e 1.58 109
Electron-transfer rate constants (kET) is calculated according to ðtCdS=MoS2 Þ1 (tCdS)1.
that of without light illuminated. This result suggests an improvement of interfacial electron transfer rate between redox couples in the solution and electrode surface under light irradiation [4,37]. Moreover, to investigate the internal resistance and interfacial charge transfer of the electrodes under dark and visible light irradiation, EIS data were analyzed, as shown in Fig. 7B. As shown, the radius of semicircle arc of Pt-CdS/MoS2 electrode was much smaller than that of Pt-CdS modified electrode under similar condition. Furthermore, when Pt-CdS/MoS2 modified electrode was under visible light illumination, the radius of semicircle arc was much smaller than that of same electrode under dark and Pt-CdS electrode under visible light illumination, suggesting a decrease in the solid state interface layer resistance and an increase of charge transfer on the surface [38,39]. The corresponding equivalent circuit was used to fit the EIS datum of Pt-CdS/MoS2 under light irradiation, shown in Fig. 7C. Where Rs represents the electrolyte resistance, Q is the electrode double-layer capacitance formed at PtCdS/MoS2/solution interface, Rct is the charge-transfer resistance at Pt-CdS/MoS2/solution interface, Ro is the chargetransfer resistance at FTO/solution interface, S is Warburg diffusion impedance and Co represents the capacitance of the composite materials on the surface of the electrode. Beside the K3 [Fe(CN)6/K4 [Fe(CN)6] as redox probe, the corresponding EIS spectra of different conditions were also measured in 1.0 M CH3OH and 1.0 M KOH solution, similar results were observed (Fig. S8). The parameters of Rct for both electrolytes were summarized in Table 3. It's more easily to see that the Pt-CdS/ MoS2 displayed smallest resistance under visible light irradiation, which is in accordance with the EIS spectra.
Fig. 7 e CVs (A) and (B) EIS spectra of Pt-CdS (a, b) and Pt-CdS/MoS2 (c, d) electrode in of in 2.5 mM K3 [Fe(CN)6/K4 [Fe(CN)6] and 0.1 M KCl solution at a potential of 0.25 V under dark (a, c) and visible light irradiation (b, d). C: Equivalent circuit was used for simulating the impedance spectrum on the Pt-CdS/MoS2 electrode under visible light irradiation. Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Table 3 e EIS fitting parameters of Rct from equivalent circuits for different samples from Fig. 7C and Fig. S8B. Electrode Rct from Fig. 7C Rct from Fig. S8B a b
Pt-CdSa
Pt-CdSb 2
680.0 U cm 7647 U cm2
Pt-CdS/MoS2a 2
290.1 U cm 2747 U cm2
2
312.4 U cm 3225.0 U cm2
Pt-CdS/MoS2b 223.7 U cm2 1530 U cm2
Without light. Under visible light irradiation.
Based on above results, CdS/MoS2 heterostructures acted as efficient photo-activated support for depositing Pt nanoclusters. Great improvements of activity and stability for MOR under visible light irradiation were achieved with this system. This is due to the synergistic effects of photocatalytic and electrocatalytic processes during catalytic methanol oxidation, as show in Scheme 2. Firstly, the methanol can be oxidized on the surface of Pt nanoclusters by traditional electrocatalytic process. At the same time, when the electrode is under visible light illumination, the CdS nanowires absorb light energy, resulting in electrons and holes. The interfacial electron transfer from the excited CdS moieties to the decorated ultrathin MoS2 shell contributes to efficient charger separation (demonstrated by Figs. 6 and 7). The holes are positive enough to oxidized OH/H2O to hydroxyl radicals ($OHs) at the surface of electrode [4e8]. These $OHs are strong oxidant species for oxidizing methanol, resulting in photooxidation of methanol at the surface of electrode [4e8]. Furthermore, the reactive radical species will oxidize the intermediate carbonaceous species like COads, which lead to an efficiently poisoning suppression [10,12]. Therein, the catalytic activity and stability of MOR on as-prepared electrode can be improved efficiently with visible light assisted. To demonstrate the generation of the $OHs in the CdS/ MoS2 during visible light irradiation, electron spin resonance (ESR) spectrum with DMPO spin trapping adducts were thus used to check the generation of $OH in the CdS/MoS2/air system. As shown in Fig. 8, four-line spectra with relative
Fig. 8 e DMPO spin-trapping electron spin resonance spectra of CdS/MoS2 nanocomposite for DMPO-OH.
intensities of 1:2:2:1 were observed in the aqueous dispersions of CdS/MoS2. This result clearly shows the generation of $OH in the presence of CdS/MoS2 under visible light irradiation [40]. These $OHs will synergistic improve the catalytic oxidization of methanol during photoelectrocatalytic process (Scheme 2).
Scheme 2 e Proposed schematic illustration for synergistic photoe and electroe catalytic MOR process by using Pt-CdS/ MoS2 core/shell nanoarchitectures modified electrode under visibleelight illumination.
Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035
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Conclusions [7]
In summary, ultrathin 2D MoS2 sheets coated 1D CdS nanowires core/shell heterostructures were synthesized by using one-step solvothermal method. After depositing Pt nanoclusters, high performances visible-light enhanced electrocatalytic oxidation of alcohol were clearly observed comparing with traditional ambient electrocatalytic oxidation. The efficient interfacial electron transfer from the excited CdS nanowires to ultrathin MoS2 shell helps to improve the synergistic effect of photocatalytic and electrocatalytic process for boosting of catalytic alcohol oxidation. The outstanding catalytic performance shows that the CdS nanowire/MoS2 nanosheet core/shell heterarchitecture could act as a promising photo-activated material in the fields of solar and chemical energy conversion and this result also provides new insight into the development of visible light photo-activated electrode for applications in fuel cells.
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Acknowledgment This work was sponsored by K.C. Wang Magna Fund in Ningbo University. The authors also appreciate to the NSFC (21603111, 51373111), Suzhou Nanoeproject (ZXG2012022), and the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207).
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Appendix A. Supplementary data [16]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.035. [17]
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Please cite this article in press as: Zhai C, et al., Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.11.035