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Formation of hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets on CdS nanorods: Toward enhanced solar-driven H2 production
Journal of Catalysis 352 (2017) 617–626
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Formation of hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets on CdS nanorods: Toward enhanced solar-driven H2 production Eun Hwa Kim, D. Amaranatha Reddy, Sangyeob Hong, Hanbit Park, Rory Ma, D. Praveen Kumar, Tae Kyu Kim ⇑ Department of Chemistry and Chemical Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 May 2017 Revised 27 June 2017 Accepted 28 June 2017
Keywords: CdS@Ba5Nb4O15/MoS2 nanohybrid Photocatalytic hydrogen High stability Renewable energy
a b s t r a c t Solar-driven semiconductor-catalyzed photocatalytic water splitting is an important and eco-friendly chemical technique for the production of clean hydrogen fuel. However, a cost-effective, efficient photocatalyst with perfect photon-to-hydrogen molecule conversion remains elusive. Novel, noble-metal-free hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets on CdS nanorods, with efficient photo-charge separation and migration capability for efficient solar-driven hydrogen production are designed. The nano-hybrid structures display a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation; this value is much higher than those of the CdS/MoS2 (124 mmolg–1h–1) and CdS/Ba5Nb4O15 (18 mmolg–1h–1) nanostructures and that of the expensive CdS/Pt benchmark catalyst (34.98 mmolg–1h–1). The apparent quantum yield at 425 nm reaches to 28.2% in 5 h. Furthermore, the rate of solar-driven hydrogen evolution in the presence of the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on the CdS nanorods is much faster than that of several noble-metal-free co-catalyst-modified CdS nanostructures reported earlier. UV–Vis absorption, photoluminescence, photocurrent, and impedance analyses of CdS@Ba5Nb4O15/MoS2 reveal that the high photocatalytic hydrogen evolution rate may due to the comparatively higher solar light-harvesting capacity and efficient charge separation and migration, which reduces the recombination rate. We anticipate that the presented design strategy for the development of noble metal-free catalysts combining perovskite and semiconductor nanostructures stimulate the development of diverse non-precious robust solar light-harvesting noble-metal-free materials for water splitting to satisfy the growing global energy demand. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Recent intensification of the energy crisis and environmental issues caused by the excessive utilization of non-renewable fossil fuels has aroused tremendous global interest in solar-driven clean energy production as the ideal choice [1]. Semiconductor photocatalytic hydrogen evolution via water splitting is considered promising for the direct capture of solar energy and conversion to chemical energy without producing environmentally harmful byproducts [2]. To achieve high photon-to-hydrogen molecule conversion efficiency in the photocatalytic reaction, highly active and stable photocatalysts are of paramount importance [3]. The photon-to-hydrogen molecule conversion efficiency is strongly ⇑ Corresponding author. E-mail address: [email protected] (T.K. Kim). http://dx.doi.org/10.1016/j.jcat.2017.06.033 0021-9517/Ó 2017 Elsevier Inc. All rights reserved.
dependent on the solar light harvesting properties of the photocatalyst, as well as the charge separation, migration, and reduction ability of the photogenerated electrons [4]. Among the wide variety of available semiconductor photocatalysts for solar-driven hydrogen production, cadmium sulfide (CdS) is widely investigated even though there are environmental and toxicity concerns, because of their superior characteristics such as adequate band-gap for harvesting solar energy, and low cost [5]. Moreover, the conduction band edge potential of CdS is more negative than the reduction potential of the H+/H2 redox couple and the valence band edge potential is more positive than the redox potential of O2/H2O; therefore, CdS preferentially reduces protons (H+) to hydrogen gas [6]. However, CdS nanostructures have inherently low photocatalytic activity for hydrogen production due to their high photo-induced charge recombination rate and photo corrosion nature which greatly limits their practical
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application [7]. To effectively retard the recombination of electronhole pairs and photo corrosion nature, formation of noble-metalbased CdS nanostructures is deemed the most efficient approach for hydrogen production, although the high price and scarcity of noble metals severely impede their practical application [8]. To overcome this difficulty, recently, several low-price, earthabundant transition metal nanostructures were developed as cocatalyst materials and were confirmed to be promising candidates for sustainable hydrogen production via water splitting [9]. Among the studied low-price, earth-abundant materials, molybdenum disulfide (MoS2) has received tremendous attention due to its large in-plane electron mobility and remarkable mechanical, electrical, and optical properties [10]. Although bulk MoS2 nanostructures are indirect band-gap semiconductors and their conduction band position is more positive than that of the H+/H2 redox couple, theoretically, only materials having a conduction band potential more negative than the H+/H2 potential can fulfill the requirement for hydrogen reduction. Thus, the electrons in the conduction band of bulk MoS2 cannot interact with H+ ions to form hydrogen gas [11]. Moreover, both theoretical and experimental studies demonstrated that the sites located along the edges of the MoS2 layers are the only active sites for the hydrogen evolution reaction, whereas the basal surfaces are catalytically inert [12,13]. Considering the two aforementioned key factors, numerous attempts have been made to improve the number of active sites or change the valence band and conduction band position to meet the requirements for the hydrogen evolution reaction and enhance the catalytic activity to achieve high quantum yields. These approaches include the incorporation of transition metal or noble metal ions (Cu, Ni, Cr, Ag), doping with heteroatoms (B, C, N, P), gradually tuning the layer numbers through exfoliation, growth on other matrices such as graphene or carbon nanotubes, and formation of heterostructures with other suitable semiconductor nanostructures [14–20]. Recently, ultrathin, layered perovskite nanostructured materials have emerged as fascinating materials for photocatalytic hydrogen evolution via water splitting under solar irradiation due to their attractive crystal structures, light harvesting capability, and suitable reduction potentials [21–24]. More interestingly, Ba5Nb4O15 (BNO), a perovskite with a hexagonal crystal structure, has demonstrated outstanding photocatalytic efficiency due to its
structural novelty [25,26]. However, the use of BNO alone in photocatalytic hydrogen evolution limits the hydrogen evolution rate due to rapid recombination of the photo-generated electron-hole pairs. Thus, it is essential to develop heterostructured photocatalysts with favorable efficiencies [27]. Considering the importance of monolayer MoS2 nanosheets and perovskite materials, herein, for the first time, we present the development of hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets as a noble-metal-free cocatalyst. The combination of Ba5Nb4O15-MoS2 nanosheets on CdS nanorods greatly improves the charge separation and migration for efficient solar-driven hydrogen production. The designed nano-hybrid nanostructures have a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation; this value is much higher than those of CdS/MoS2 and CdS/Ba5Nb4O15 nanostructures and that of the expensive CdS/Pt benchmark catalyst. Furthermore, the solar-driven hydrogen evolution rate achieved with the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on CdS nanorods is much larger than that of several noble-metal-free cocatalyst-modified CdS nanostructures reported earlier. We believe that the proposed design strategy should pave the way for developing other highly active noble-metal-free photocatalysts for water splitting applications. 2. Results and discussion The synthesis process of the CdS/BNO-MoS2 nanocomposites is schematically presented in Scheme 1. The detailed experimental procedures for the syntheses of CdS, CdS/MoS2, CdS/Ba5Nb4O15 and CdS/BNO-MoS2 nanocomposites are provided in the experimental section in Supporting Information (SI). The morphology and crystal structure of as-synthesized Ba5Nb4O15, MoS2, and the Ba5Nb4O15/MoS2 nanocomposite were analyzed in detail by using transmission electron microscopy (TEM) and high resolution-TEM (HRTEM). Fig. 1(a) shows a TEM image of Ba5Nb4O15 synthesized via the hydrothermal method at 230 °C for 12 h under air atmosphere. Fig. 1(a) clearly shows twodimensional (2D) ultrathin nanosheets of Ba5Nb4O15 with a wall thickness of about 25 nm and an average width of about 250 nm, having a very smooth surface with very high crystallinity. The
Scheme 1. Schematic illustration of the process of formation of CdS/Ba5Nb4O15-MoS2 nanocomposites.
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Fig. 1. TEM and HRTEM images of (a, b) Ba5Nb4O15 nanosheets, (c, d) MoS2 nanosheets and (e, f) Ba5Nb4O15-MoS2 nanostructures.
HR-TEM image in Fig. 1(b) shows clear lattice fringes, indicating good crystallinity of the synthesized Ba5Nb4O15 nanosheets; lattice fringes of 0.58 and 0.29 nm were observed, corresponding to the (1 0 0) and (1 1 0) planes of hexagonal-structured Ba5Nb4O15 (JCPDS 14-0028). Fig. 1(c) shows a TEM image of the assynthesized MoS2, clearly revealing its layered structure. Additionally, the HR-TEM image in Fig. 1(d) shows that these nanostructures have an interlayer spacing of 0.62 nm and were 14 layers thick. Fig. 1(e) shows that the Ba5Nb4O15/MoS2 nanocomposites comprised ultrathin MoS2 nanosheets on 2D ultrathin Ba5Nb4O15 nanosheets. The magnified TEM image in Fig. 1(f) and Fig. S1 reveals that the ultrathin layered MoS2 nanosheets have a lattice fringe spacing of 0.62 nm and the Ba5Nb4O15 nanosheets have lattice fringe spacings of 0.585 and 0.292 nm. The HAADF-STEM image further revealed the composite nature of the Ba5Nb4O15/ MoS2 nanostructures and the elemental distribution of the
nanocomposite (Fig. 2(a h)). The elemental mapping patterns (Fig. 2(b g)) show the presence of Mo, S, Ba, Nb, and O in the synthesized Ba5Nb4O15/MoS2 nanocomposite. Further, the line profiles of the Ba5Nb4O15/MoS2 nanocomposite indicate that the nanocomposite is a mixture of MoS2 and Ba5Nb4O15 nanostructures. The crystal structural features of the as-synthesized MoS2, Ba5Nb4O15, and Ba5Nb4O15/MoS2 nanostructures were analyzed by powder X-ray diffraction (XRD; Fig. S2). The XRD pattern of the pure as-synthesized Ba5Nb4O15 nanosheets could be indexed to the hexagonal structure (JCPDS 14-0028) with good crystallinity. The XRD patterns of the as-synthesized MoS2 nanosheets exhibit broad diffraction peaks that were readily indexed to the hexagonal phase based on the high consistency with JCPDS card number 371492. The major diffraction peak around 14° corresponds to the lattice (0 0 2) plane with a d-spacing of 0.62 nm, indicating a well-stacked layered structure along the c axis. The XRD patterns
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of the Ba5Nb4O15/MoS2 nanostructures confirm the formation of a nanocomposite with diffraction peaks of both the Ba5Nb4O15 and MoS2 nanostructures. The optical properties and light-harvesting properties of MoS2, Ba5Nb4O15, and the Ba5Nb4O15/MoS2 nanostructures were investigated by UV vis diffuse reflectance spectroscopy (DRS; Fig. S3). The optical absorption maximum of Ba5Nb4O15 was observed at 342 nm, which is consistent with earlier reports [25]. The absorp-
tion spectrum of the as-synthesized MoS2 nanosheets (Fig. S3) revealed well-known excitonic absorption bands around 688 and 629 nm, which are related to the spin-orbit splitting of the top of the valence band at the K-point of the Brillouin zone in 2D MoS2, originating from the bonding interaction of the laterally extended Mo 4d orbitals with S 3p orbitals [28–34]. The Ba5Nb4O15/MoS2 nanocomposites showed higher absorption intensity in the wavelength range of 400–800 nm, compared to the pure Ba5Nb4O15
Fig. 2. (a) HAADF electron micrograph and (b-g) elemental mapping of Ba5Nb4O15-MoS2 nanocomposite, showing the presence of (b) Ba, (c) S, (d) Nb, (e) Mo, (f) O, (g) all elements, and (h) line spectra of elements present, respectively.
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nanostructures, which is beneficial to the photocatalytic activity of the composite for enhanced harvesting of the incident light. To evaluate the morphology, the elemental distribution, and the interfacial contacts between MoS2, Ba5Nb4O15, and CdS, the CdS@Ba5Nb4O15/MoS2 photocatalyst was characterized by TEM and HRTEM with energy dispersive spectroscopy (EDS) and the obtained results are presented in Fig. 3(a i). The micrographs reveal that the layered MoS2 and 2D ultrathin Ba5Nb4O15 nanosheets were attached to the CdS nanorods, forming intimate interfacial contact with each nanostructure (Fig. 3(a)). The HRTEM image in Fig. 3(b) and Fig. S4 reveals that the ultrathin layered MoS2 nanosheets have a lattice fringe spacing of 0.62 nm, corresponding to the (0 0 2) plane of hexagonal MoS2. The Ba5Nb4O15 nanosheets have lattice fringe spacings of 0.292 nm, corresponding to the (0 0 2) plane of hexagonal Ba5Nb4O15.The CdS nanorods have a lattice fringe spacing of 0.29 nm, corresponding to the (1 0 1) plane of hexagonal CdS. For comparison, the hydrothermallysynthesized bare CdS nanostructures are also presented in Fig. S5, clearly showing nanostructures having a rod-shaped morphology with a length of 200–400 nm and width of 5–10 nm. Further, the HAADF-STEM image and EDS mapping analyses of the CdS@Ba5Nb4O15/MoS2 nanocomposites confirmed the distribution of Cd, S, Ba, Nb, O, and Mo in the system (Fig. 3(c i)). The EDS results obtained for CdS and the CdS@Ba5Nb4O15/MoS2 nanocom-
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posite (Fig. S6 (a, b)) indicate the presence of Cd and S in bare CdS, whereas Mo, Ba, Nb, and O were detected in the nanocomposites. The TEM, HRTEM EDS, and EDS mapping micrographs clearly confirm formation of strong, intimate contacts between Ba5Nb4O15, MoS2, and the CdS nanostructures, which facilitates much more efficient transfer of photo-generated electrons between adjacent nanostructures, and therefore enhances the photocatalytic hydrogen evolution rate. The phase purity and crystallinity of the as-synthesized CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS@Ba5Nb4O15/MoS2 nanostructures were investigated by X-ray diffraction (XRD). As shown in Fig. 4(a), the diffraction patterns of the pure CdS nanorods and their nanocomposites correspond to the hexagonal structure (JCPDS card No. 89-2944), with no typical diffraction peaks of the MoS2 and Ba5Nb4O15 nanostructures, which is ascribed weak diffraction intensity of the Ba5Nb4O15 and MoS2 nanostructures [11,14]. In order to evaluate the state of the elements and formation of the CdS@Ba5Nb4O15/MoS2 nanocomposite, X-ray photoelectron spectroscopy (XPS) analysis was carried out (Fig. 4(b–f)). Generally, XPS is a powerful tool for investigating the surface composition and electronic structure of materials and can provide information about the chemical environment of the elements present in the synthesized nanocomposite. The survey spectrum in Fig. 4(b)
Fig. 3. (a, b) TEM and HRTEM electron micrograph and elemental mapping of CdS/Ba5Nb4O15-MoS2 nanocomposite, showing the presence of (d) Cd, (e) S, (f) Mo, (g) Ba, (h) Nb, and (i) oxygen, respectively.
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Fig. 4. (a) XRD patterns of CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS/Ba5Nb4O15/MoS2 nanostructures. (b) XPS survey spectrum of CdS/Ba5Nb4O15/MoS2 nanocomposite. ((c)– (h)) Narrow-scan spectra of Cd 3d, S 2p, Mo 3d, Ba 3d, Nb 3d, and O 1s species in CdS/Ba5Nb4O15-MoS2 nanocomposite.
clearly shows the presence of Cd 3d, S 2p, Mo 3d, Ba 3D, Nb 3D, and O 1s peaks of the elements in the CdS@Ba5Nb4O15/MoS2 nanocomposite. The oxidation states of these elements were confirmed from the high-resolution XPS spectra. Fig. 4(c) shows the narrow-scan XPS spectrum of Cd 3d, with peaks at binding energies of 404.96 and 411.73 eV, corresponding to Cd3d5/2 and Cd3d3/2 transitions, respectively [35]. The data confirm that Cd is in the di-positive oxidation state in CdS. Fig. 4(d) shows the narrow-scan XPS spectrum with S2p peaks at binding energies of 161.1 and 162.31 eV [35]. The high-resolution Mo 3d spectrum in Fig. 4(e) shows two peaks at 232.1 and 228.0 eV, which are attributed to the Mo 3d3/2 and Mo 3d5/2 states and confirmed that Mo4+ was the dominant oxidation state. Further, the small peak at the binding energy of 236 eV
was ascribed to the presence of a small amount of Mo6+ [36]. The high-resolution Ba 3d spectrum presented in Fig. 4(f) reveals the divalent oxidation state of Ba, indicated by peaks at 780.6 and 795.5 eV, which are assigned to Ba 3d3/2 and Ba 3d5/2, respectively [37]. The narrow-scan Nb 3d XPS spectrum shows peaks at binding energies of 206.43 and 209.72 eV, which correspond to Nb 3d3/2 and Nb 3d5/2, respectively (Fig. 4(g)) [38]. The O 1s narrow-scan spectrum of the CdS@Ba5Nb4O15/MoS2 composite shown in Fig. 4 (h) could be deconvoluted into three peaks at binding energies of 530.69, 531.94, and 533.3 eV, corresponding to lattice oxygen (OL), defect/vacancy oxygen (OV), and chemisorbed oxygen (OC), respectively [39]. The XPS results unambiguously confirm successful formation of the CdS@Ba5Nb4O15/MoS2 nanocomposites. The
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Fig. 5. (a) DRS, (b) PL, (c) photocurrent, and (d) impedance spectra of CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS/Ba5Nb4O15/MoS2 nanostructures. Mott-Schottky plots of (e) CdS nanorods and (f) BNO-MoS2 nanostructures.
optical band-gap and light-harvesting capability of CdS and the nanocomposites were investigated by UV vis diffuse reflectance spectroscopy (Fig. 5(a)). The absorption maximum of bare CdS was located around 540 nm, and the compound was thus capable of harvesting visible light. The addition of MoS2 and Ba5Nb4O15/ MoS2 increased the absorption intensity of CdS in the 550– 800 nm range, which is beneficial for improving the quantum efficiency of the CdS@Ba5Nb4O15/MoS2 composites. The estimated band-gap values for CdS and the nanocomposites were in the range of 2.41–2.415 eV. To understand the mechanism of recombination of the photogenerated charge carriers and the contribution of MoS2 and Ba5Nb4O15 to promoting transfer of the photo-generated electrons and charge separation, the photoluminescence (PL) emission was analyzed as shown in Fig. 5(b). The as-synthesized CdS nanostructures showed a strong emission band with a maximum around 550 nm, ascribed to the surface defects and vacancies [40]. Whereas the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/ MoS2 nanocomposites exhibited peaks at similar positions, the intensity of the composite peaks was much lower than that of
the bare CdS nanostructures. In particular, the CdS@Ba5Nb4O15/ MoS2 nanocomposite showed the lowest PL intensity relative to the other synthesized nanocomposites, indicating that Ba5Nb4O15/ MoS2 promotes transfer of the photo-generated electrons and charge separation in CdS, thereby enhancing the electron-hole pair separation. To further confirm the enhanced charge carrier transport and separation efficiency, the transient photocurrents and impedance of CdS and the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/ MoS2 nanocomposites were photo-electrochemically examined under simulated solar irradiation. Fig. 5(c) shows the photocurrent responses of bare CdS and the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/MoS2 nanocomposites with 30 s on/off irradiation cycles. As anticipated, the CdS@Ba5Nb4O15/MoS2 nanocomposites showed the highest photocurrent intensity with respect to time compared with the bare CdS nanostructures and corresponding nanocomposites, owing to the suitable band structure for tunneling photo-generated electrons and holes and higher charge transfer and separation of the former. Moreover, the photocurrent responses with respect to time were highly reproducible for sev-
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Fig. 6. (a) Amount of hydrogen evolved in 15 mL of aqueous solution containing lactic acid (3 mL) and DI water (12 mL), with suspended CdS/Ba5Nb4O15/MoS2 nanostructures (1 mg) containing 0, 1, 3, 5, 6, 7, and 10 wt% Ba5Nb4O15/MoS2 nanostructures. (b) Comparison of the optimized photocatalyst with other catalysts (1: CdS, 2: B-BNO/CdS, 3: UT-BNO/CdS, 4: CdS/Pt, 5: CdS/B-MoS2, 6: UT-MoS2, and 7: UT-MoS2-BNO/CdS). (c) Effect of photocatalyst loading on hydrogen production rate (photocatalyst: CdS/WS2MoS2 (6 wt%) nanocomposite; photocatalyst loading: 1, 3, 5, and 7 mg; time: 5 h; water/lactic acid: 12:3 mL). (d) Effect of concentration of sacrificial lactic acid on the hydrogen evolution rate (photocatalyst: CdS/Ba5Nb4O15/MoS2 nanocomposite; photocatalyst dose: 1 mg; time: 5 h). (e) Hydrogen production in recycling experiments using the optimized CdS/Ba5Nb4O15-MoS2 and CdS/MoS2 nanocomposites. (f) Long-term stability of the optimized CdS/Ba5Nb4O15-MoS2 nanocomposites.
eral on/off cycles and remained stable, demonstrating that the presence of MoS2 and Ba5Nb4O15 on the CdS nanorods can effectively prevent photocorrosion of CdS [41]. Furthermore, electrochemical impedance spectral (EIS) analysis (Fig. 5(d)) showed a smaller semicircular region for the CdS@Ba5Nb4O15/MoS2 nanocomposite than the other nanocomposites and bare CdS nanostructures, demonstrating that CdS@Ba5Nb4O15/MoS2 exhibited the fastest interfacial charge transfer due to the MoS2 and Ba5Nb4O15 co-catalysts [42]. Based on the above experimental data, it is apparent that embedding MoS2 and Ba5Nb4O15 on CdS can effectively enhance the rate of photocatalytic hydrogen evolution by the CdS nanostructures. In order to evaluate the feasibility of transfer of the photogenerated electrons from the CdS nanostructures to the Ba5Nb4O15/ MoS2 nanocomposites, the flat-band potential (VFB) was estimated by using Mott Schottky plots at a frequency of 1 kHz. The estimated conduction band potentials of the CdS and Ba5Nb4O15/
MoS2 nanocomposites were 1.12 and 0.74 V vs. Ag/AgCl, respectively (Fig. 5(e, f)). The results demonstrated that the estimated conduction band potential of Ba5Nb4O15/MoS2 is more positive than that of the CdS nanostructures and more negative than that of the H+/H2 redox couple ( 0.41 V vs. NHE at pH = 7) [43]. Hence, Ba5Nb4O15/MoS2 served as a co-catalyst and contributed to efficient solar-driven hydrogen evolution. The photocatalytic activity of hydrogen was evaluated by using lactic acid as a hole scavenger under simulated solar irradiation. Here, we used an air-tight reaction quartz beaker that was externally irradiated with a solar simulator. The evolved hydrogen was sampled with a gas-tight syringe and analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector. As depicted in Fig. 6(a), the as-synthesized bare CdS nanostructures generated a very low amount of hydrogen (2.54 mmol h–1 g– 1 ) due to the fast recombination of the electron–hole pairs [44]. For comparison bare Ba5Nb4O15 and Ba5Nb4O15/MoS2 nanocatalysts
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also tested for hydrogen evolution and we noticed very little hydrogen was generated using pure MoS2 (0.04 mmol h 1 g 1) and Ba5Nb4O15/MoS2 (0.58 mmol h 1 g 1) nanostructures, indicating that bare Ba5Nb4O15 and Ba5Nb4O15/MoS2 nanostructures are not efficient for photocatalytic hydrogen evolution. The CdS nanocrystals were incorporated with MoS2/Ba5Nb4O15 and the loading of MoS2/Ba5Nb4O15 on CdS was optimized to maximize the hydrogen production. Notably, the presence of a small amount of MoS2/Ba5Nb4O15 on CdS led to greater hydrogen production and increased with increasing loading of MoS2/Ba5Nb4O15 on CdS up to the optimum level of 6.0 wt%, with a decrease at higher loadings (Fig. 6(a)). The reduced rate of hydrogen production at high loading may be due to the increased coverage of the CdS surface by MoS2/ Ba5Nb4O15, which reduces the area of CdS exposed to light and consequently lowers the rate of electron-hole pair generation [45]. The photocatalyst loading and hole scavenger concentration were also optimized; 1 mg of catalyst and 20 vol% of hole scavenger were suitable for high hydrogen production (Fig. 6(c, d)). Under the optimized conditions, the maximum rate of hydrogen evolution was 147 mmolh 1g 1, achieved using CdS@MoS2/Ba5Nb4O15 (UTMoS2-BNO/CdS). This rate is significantly higher than that achieved with CdS, CdS@ Ba5Nb4O15 (B-BNO/CdS), ultrathin CdS@Ba5Nb4O15 (UT-BNO/CdS), MoS2/CdS (B-MoS2/CdS), ultrathin MoS2/CdS (UTMoS2/CdS) and CdS/Pt (Fig. 6(b)). Furthermore, the observed rate of hydrogen evolution in the presence of CdS@MoS2/Ba5Nb4O15 was much faster than that achieved with several cocatalystmodified CdS nanostructures reported earlier (Table S1). In addition to the rate of photocatalytic of hydrogen evolution, the apparent quantum efficiency (QEs) is another important parameter for assessing the photocatalytic activity. The optimized CdS@MoS2/ Ba5Nb4O15 nanocomposite was evaluated by visible-light irradiation using a 150 W Xe lamp with a 425 nm band-pass filter, and the quantum yield was estimated to be around 28.2%. Compared to the photocatalytic activity, the catalyst stability is more important in terms of practical applications. In order to verify the stability of the optimized CdS@MoS2/Ba5Nb4O15 nanostructures, recycling experiments were carried out as shown in in Fig. 6(e). From Fig. 6(e), it is clear that almost identical amounts of hydrogen were produced in all five recycling experiments. However, there was a minor decrease of the amount of hydrogen generated in the second and third cycles compared to the first cycle, which is attributed to the decreased concentration of lactic acid due to its conversion to pyruvic acid during the reaction. To verify this speculation, an additional 3 mL of lactic acid solution was added, and the hydrogen evolution rate in the fourth and fifth cycle was similar to that in the first cycle, indicating that the catalyst was stable over five cycles. Furthermore, we have noticed that the CdS@MoS2/Ba5Nb4O15 nanostructures are more stable than the CdS/MoS2 nanostructures. Furthermore, the CdS@MoS2/Ba5Nb4O15 nanostructures showed remarkable long-term photostability up to 40 h (Fig. 6(f)), indicating that the nanocomposite is a stable, noble-metal-free catalyst for hydrogen production. The enhancement of the photocatalytic hydrogen evolution performance and the extraordinary stability of the as-synthesized CdS@MoS2/Ba5Nb4O15 nanocomposites can be attributed to the efficient MoS2/Ba5Nb4O15 co-catalyst system, having lightharvesting capability and suitable reduction potentials with rich active sites. Based on the DRS, PL, photocurrent, and impedance analyses, it is concluded that the enhanced rate of photocatalytic hydrogen evolution in the presence of the CdS@MoS2/Ba5Nb4O15 nanocomposites relative to the bare CdS nanostructures can be attributed to the following: (i) the synthesized CdS@MoS2/Ba5Nb4O15 nanocomposites have more prominent sunlight-harvesting capability than the as-synthesized CdS nanostructures. These peculiar properties may facilitate the production of photogenerated electrons from the nanocomposite, leading to enhancement
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Scheme 2. Schematic illustration of the proposed mechanism for utilization of the CdS/Ba5Nb4O15-MoS2 nanocomposites as photocatalysts for hydrogen production under simulated solar irradiation using lactic acid as a sacrificial agent.
of the photocatalytic hydrogen evolution rate. (ii) The strong, intimate contact between Ba5Nb4O15, MoS2, and the CdS nanostructures contributes to much more efficient transfer of the photogenerated electrons between adjacent nanostructures, providing more active reaction sites for efficient hydrogen production. (iii) The suitable band-edge potentials of CdS and Ba5Nb4O15-MoS2 enhance the reduction ability of the photo-excited electrons and are more favorable for photocatalytic hydrogen generation [46– 49]. A schematic summary of the proposed mechanism for utilization of the CdS/Ba5Nb4O15-MoS2 nanocomposites as photocatalysts for hydrogen production under simulated solar irradiation using lactic acid as a sacrificial agent is presented in Scheme 2. 3. Conclusions Hybrid nanostructures combining perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets embedded with CdS nanorods were prepared via a simple hydrothermal method using an ultrasonicationassisted chemical process. The resulting nanostructures were characterized by strong, intimate contact between Ba5Nb4O15, MoS2, and the CdS nanostructures and exhibited accessible charge transfer with high visible-light harvesting capability. The designed nano-hybrid nanostructures gave rise to a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation. This value is much higher than those of the CdS/MoS2 and CdS/Ba5Nb4O15 nanostructures and that of the expensive CdS/Pt benchmark catalyst. Furthermore, the rate of solar-driven hydrogen evolution in the presence of the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on the CdS nanorods is much faster than that of several noble-metal-free co-catalyst-modified CdS nanostructures reported earlier. The proposed designed strategy should pave the way for developing other highly active noble-metal-free photocatalysts for water splitting applications. Acknowledgements This work was financially supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (2014R1A4A1001690 and 2016R1E1A1A01941978). This research was also supported in part by the Max Planck POSTECH/KOREA Research Initiative Program [Grant No. 2016 K1A4A4A01922028] through the MEST’s NRF funding.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2017.06.033.
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Formation of hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets on CdS nanorods: Toward enhanced solar-driven H2 production Eun Hwa Kim, D. Amaranatha Reddy, Sangyeob Hong, Hanbit Park, Rory Ma, D. Praveen Kumar, Tae Kyu Kim ⇑ Department of Chemistry and Chemical Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 May 2017 Revised 27 June 2017 Accepted 28 June 2017
Keywords: CdS@Ba5Nb4O15/MoS2 nanohybrid Photocatalytic hydrogen High stability Renewable energy
a b s t r a c t Solar-driven semiconductor-catalyzed photocatalytic water splitting is an important and eco-friendly chemical technique for the production of clean hydrogen fuel. However, a cost-effective, efficient photocatalyst with perfect photon-to-hydrogen molecule conversion remains elusive. Novel, noble-metal-free hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets on CdS nanorods, with efficient photo-charge separation and migration capability for efficient solar-driven hydrogen production are designed. The nano-hybrid structures display a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation; this value is much higher than those of the CdS/MoS2 (124 mmolg–1h–1) and CdS/Ba5Nb4O15 (18 mmolg–1h–1) nanostructures and that of the expensive CdS/Pt benchmark catalyst (34.98 mmolg–1h–1). The apparent quantum yield at 425 nm reaches to 28.2% in 5 h. Furthermore, the rate of solar-driven hydrogen evolution in the presence of the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on the CdS nanorods is much faster than that of several noble-metal-free co-catalyst-modified CdS nanostructures reported earlier. UV–Vis absorption, photoluminescence, photocurrent, and impedance analyses of CdS@Ba5Nb4O15/MoS2 reveal that the high photocatalytic hydrogen evolution rate may due to the comparatively higher solar light-harvesting capacity and efficient charge separation and migration, which reduces the recombination rate. We anticipate that the presented design strategy for the development of noble metal-free catalysts combining perovskite and semiconductor nanostructures stimulate the development of diverse non-precious robust solar light-harvesting noble-metal-free materials for water splitting to satisfy the growing global energy demand. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Recent intensification of the energy crisis and environmental issues caused by the excessive utilization of non-renewable fossil fuels has aroused tremendous global interest in solar-driven clean energy production as the ideal choice [1]. Semiconductor photocatalytic hydrogen evolution via water splitting is considered promising for the direct capture of solar energy and conversion to chemical energy without producing environmentally harmful byproducts [2]. To achieve high photon-to-hydrogen molecule conversion efficiency in the photocatalytic reaction, highly active and stable photocatalysts are of paramount importance [3]. The photon-to-hydrogen molecule conversion efficiency is strongly ⇑ Corresponding author. E-mail address: [email protected] (T.K. Kim). http://dx.doi.org/10.1016/j.jcat.2017.06.033 0021-9517/Ó 2017 Elsevier Inc. All rights reserved.
dependent on the solar light harvesting properties of the photocatalyst, as well as the charge separation, migration, and reduction ability of the photogenerated electrons [4]. Among the wide variety of available semiconductor photocatalysts for solar-driven hydrogen production, cadmium sulfide (CdS) is widely investigated even though there are environmental and toxicity concerns, because of their superior characteristics such as adequate band-gap for harvesting solar energy, and low cost [5]. Moreover, the conduction band edge potential of CdS is more negative than the reduction potential of the H+/H2 redox couple and the valence band edge potential is more positive than the redox potential of O2/H2O; therefore, CdS preferentially reduces protons (H+) to hydrogen gas [6]. However, CdS nanostructures have inherently low photocatalytic activity for hydrogen production due to their high photo-induced charge recombination rate and photo corrosion nature which greatly limits their practical
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application [7]. To effectively retard the recombination of electronhole pairs and photo corrosion nature, formation of noble-metalbased CdS nanostructures is deemed the most efficient approach for hydrogen production, although the high price and scarcity of noble metals severely impede their practical application [8]. To overcome this difficulty, recently, several low-price, earthabundant transition metal nanostructures were developed as cocatalyst materials and were confirmed to be promising candidates for sustainable hydrogen production via water splitting [9]. Among the studied low-price, earth-abundant materials, molybdenum disulfide (MoS2) has received tremendous attention due to its large in-plane electron mobility and remarkable mechanical, electrical, and optical properties [10]. Although bulk MoS2 nanostructures are indirect band-gap semiconductors and their conduction band position is more positive than that of the H+/H2 redox couple, theoretically, only materials having a conduction band potential more negative than the H+/H2 potential can fulfill the requirement for hydrogen reduction. Thus, the electrons in the conduction band of bulk MoS2 cannot interact with H+ ions to form hydrogen gas [11]. Moreover, both theoretical and experimental studies demonstrated that the sites located along the edges of the MoS2 layers are the only active sites for the hydrogen evolution reaction, whereas the basal surfaces are catalytically inert [12,13]. Considering the two aforementioned key factors, numerous attempts have been made to improve the number of active sites or change the valence band and conduction band position to meet the requirements for the hydrogen evolution reaction and enhance the catalytic activity to achieve high quantum yields. These approaches include the incorporation of transition metal or noble metal ions (Cu, Ni, Cr, Ag), doping with heteroatoms (B, C, N, P), gradually tuning the layer numbers through exfoliation, growth on other matrices such as graphene or carbon nanotubes, and formation of heterostructures with other suitable semiconductor nanostructures [14–20]. Recently, ultrathin, layered perovskite nanostructured materials have emerged as fascinating materials for photocatalytic hydrogen evolution via water splitting under solar irradiation due to their attractive crystal structures, light harvesting capability, and suitable reduction potentials [21–24]. More interestingly, Ba5Nb4O15 (BNO), a perovskite with a hexagonal crystal structure, has demonstrated outstanding photocatalytic efficiency due to its
structural novelty [25,26]. However, the use of BNO alone in photocatalytic hydrogen evolution limits the hydrogen evolution rate due to rapid recombination of the photo-generated electron-hole pairs. Thus, it is essential to develop heterostructured photocatalysts with favorable efficiencies [27]. Considering the importance of monolayer MoS2 nanosheets and perovskite materials, herein, for the first time, we present the development of hybrid nanostructures comprising perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets as a noble-metal-free cocatalyst. The combination of Ba5Nb4O15-MoS2 nanosheets on CdS nanorods greatly improves the charge separation and migration for efficient solar-driven hydrogen production. The designed nano-hybrid nanostructures have a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation; this value is much higher than those of CdS/MoS2 and CdS/Ba5Nb4O15 nanostructures and that of the expensive CdS/Pt benchmark catalyst. Furthermore, the solar-driven hydrogen evolution rate achieved with the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on CdS nanorods is much larger than that of several noble-metal-free cocatalyst-modified CdS nanostructures reported earlier. We believe that the proposed design strategy should pave the way for developing other highly active noble-metal-free photocatalysts for water splitting applications. 2. Results and discussion The synthesis process of the CdS/BNO-MoS2 nanocomposites is schematically presented in Scheme 1. The detailed experimental procedures for the syntheses of CdS, CdS/MoS2, CdS/Ba5Nb4O15 and CdS/BNO-MoS2 nanocomposites are provided in the experimental section in Supporting Information (SI). The morphology and crystal structure of as-synthesized Ba5Nb4O15, MoS2, and the Ba5Nb4O15/MoS2 nanocomposite were analyzed in detail by using transmission electron microscopy (TEM) and high resolution-TEM (HRTEM). Fig. 1(a) shows a TEM image of Ba5Nb4O15 synthesized via the hydrothermal method at 230 °C for 12 h under air atmosphere. Fig. 1(a) clearly shows twodimensional (2D) ultrathin nanosheets of Ba5Nb4O15 with a wall thickness of about 25 nm and an average width of about 250 nm, having a very smooth surface with very high crystallinity. The
Scheme 1. Schematic illustration of the process of formation of CdS/Ba5Nb4O15-MoS2 nanocomposites.
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Fig. 1. TEM and HRTEM images of (a, b) Ba5Nb4O15 nanosheets, (c, d) MoS2 nanosheets and (e, f) Ba5Nb4O15-MoS2 nanostructures.
HR-TEM image in Fig. 1(b) shows clear lattice fringes, indicating good crystallinity of the synthesized Ba5Nb4O15 nanosheets; lattice fringes of 0.58 and 0.29 nm were observed, corresponding to the (1 0 0) and (1 1 0) planes of hexagonal-structured Ba5Nb4O15 (JCPDS 14-0028). Fig. 1(c) shows a TEM image of the assynthesized MoS2, clearly revealing its layered structure. Additionally, the HR-TEM image in Fig. 1(d) shows that these nanostructures have an interlayer spacing of 0.62 nm and were 14 layers thick. Fig. 1(e) shows that the Ba5Nb4O15/MoS2 nanocomposites comprised ultrathin MoS2 nanosheets on 2D ultrathin Ba5Nb4O15 nanosheets. The magnified TEM image in Fig. 1(f) and Fig. S1 reveals that the ultrathin layered MoS2 nanosheets have a lattice fringe spacing of 0.62 nm and the Ba5Nb4O15 nanosheets have lattice fringe spacings of 0.585 and 0.292 nm. The HAADF-STEM image further revealed the composite nature of the Ba5Nb4O15/ MoS2 nanostructures and the elemental distribution of the
nanocomposite (Fig. 2(a h)). The elemental mapping patterns (Fig. 2(b g)) show the presence of Mo, S, Ba, Nb, and O in the synthesized Ba5Nb4O15/MoS2 nanocomposite. Further, the line profiles of the Ba5Nb4O15/MoS2 nanocomposite indicate that the nanocomposite is a mixture of MoS2 and Ba5Nb4O15 nanostructures. The crystal structural features of the as-synthesized MoS2, Ba5Nb4O15, and Ba5Nb4O15/MoS2 nanostructures were analyzed by powder X-ray diffraction (XRD; Fig. S2). The XRD pattern of the pure as-synthesized Ba5Nb4O15 nanosheets could be indexed to the hexagonal structure (JCPDS 14-0028) with good crystallinity. The XRD patterns of the as-synthesized MoS2 nanosheets exhibit broad diffraction peaks that were readily indexed to the hexagonal phase based on the high consistency with JCPDS card number 371492. The major diffraction peak around 14° corresponds to the lattice (0 0 2) plane with a d-spacing of 0.62 nm, indicating a well-stacked layered structure along the c axis. The XRD patterns
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of the Ba5Nb4O15/MoS2 nanostructures confirm the formation of a nanocomposite with diffraction peaks of both the Ba5Nb4O15 and MoS2 nanostructures. The optical properties and light-harvesting properties of MoS2, Ba5Nb4O15, and the Ba5Nb4O15/MoS2 nanostructures were investigated by UV vis diffuse reflectance spectroscopy (DRS; Fig. S3). The optical absorption maximum of Ba5Nb4O15 was observed at 342 nm, which is consistent with earlier reports [25]. The absorp-
tion spectrum of the as-synthesized MoS2 nanosheets (Fig. S3) revealed well-known excitonic absorption bands around 688 and 629 nm, which are related to the spin-orbit splitting of the top of the valence band at the K-point of the Brillouin zone in 2D MoS2, originating from the bonding interaction of the laterally extended Mo 4d orbitals with S 3p orbitals [28–34]. The Ba5Nb4O15/MoS2 nanocomposites showed higher absorption intensity in the wavelength range of 400–800 nm, compared to the pure Ba5Nb4O15
Fig. 2. (a) HAADF electron micrograph and (b-g) elemental mapping of Ba5Nb4O15-MoS2 nanocomposite, showing the presence of (b) Ba, (c) S, (d) Nb, (e) Mo, (f) O, (g) all elements, and (h) line spectra of elements present, respectively.
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nanostructures, which is beneficial to the photocatalytic activity of the composite for enhanced harvesting of the incident light. To evaluate the morphology, the elemental distribution, and the interfacial contacts between MoS2, Ba5Nb4O15, and CdS, the CdS@Ba5Nb4O15/MoS2 photocatalyst was characterized by TEM and HRTEM with energy dispersive spectroscopy (EDS) and the obtained results are presented in Fig. 3(a i). The micrographs reveal that the layered MoS2 and 2D ultrathin Ba5Nb4O15 nanosheets were attached to the CdS nanorods, forming intimate interfacial contact with each nanostructure (Fig. 3(a)). The HRTEM image in Fig. 3(b) and Fig. S4 reveals that the ultrathin layered MoS2 nanosheets have a lattice fringe spacing of 0.62 nm, corresponding to the (0 0 2) plane of hexagonal MoS2. The Ba5Nb4O15 nanosheets have lattice fringe spacings of 0.292 nm, corresponding to the (0 0 2) plane of hexagonal Ba5Nb4O15.The CdS nanorods have a lattice fringe spacing of 0.29 nm, corresponding to the (1 0 1) plane of hexagonal CdS. For comparison, the hydrothermallysynthesized bare CdS nanostructures are also presented in Fig. S5, clearly showing nanostructures having a rod-shaped morphology with a length of 200–400 nm and width of 5–10 nm. Further, the HAADF-STEM image and EDS mapping analyses of the CdS@Ba5Nb4O15/MoS2 nanocomposites confirmed the distribution of Cd, S, Ba, Nb, O, and Mo in the system (Fig. 3(c i)). The EDS results obtained for CdS and the CdS@Ba5Nb4O15/MoS2 nanocom-
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posite (Fig. S6 (a, b)) indicate the presence of Cd and S in bare CdS, whereas Mo, Ba, Nb, and O were detected in the nanocomposites. The TEM, HRTEM EDS, and EDS mapping micrographs clearly confirm formation of strong, intimate contacts between Ba5Nb4O15, MoS2, and the CdS nanostructures, which facilitates much more efficient transfer of photo-generated electrons between adjacent nanostructures, and therefore enhances the photocatalytic hydrogen evolution rate. The phase purity and crystallinity of the as-synthesized CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS@Ba5Nb4O15/MoS2 nanostructures were investigated by X-ray diffraction (XRD). As shown in Fig. 4(a), the diffraction patterns of the pure CdS nanorods and their nanocomposites correspond to the hexagonal structure (JCPDS card No. 89-2944), with no typical diffraction peaks of the MoS2 and Ba5Nb4O15 nanostructures, which is ascribed weak diffraction intensity of the Ba5Nb4O15 and MoS2 nanostructures [11,14]. In order to evaluate the state of the elements and formation of the CdS@Ba5Nb4O15/MoS2 nanocomposite, X-ray photoelectron spectroscopy (XPS) analysis was carried out (Fig. 4(b–f)). Generally, XPS is a powerful tool for investigating the surface composition and electronic structure of materials and can provide information about the chemical environment of the elements present in the synthesized nanocomposite. The survey spectrum in Fig. 4(b)
Fig. 3. (a, b) TEM and HRTEM electron micrograph and elemental mapping of CdS/Ba5Nb4O15-MoS2 nanocomposite, showing the presence of (d) Cd, (e) S, (f) Mo, (g) Ba, (h) Nb, and (i) oxygen, respectively.
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Fig. 4. (a) XRD patterns of CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS/Ba5Nb4O15/MoS2 nanostructures. (b) XPS survey spectrum of CdS/Ba5Nb4O15/MoS2 nanocomposite. ((c)– (h)) Narrow-scan spectra of Cd 3d, S 2p, Mo 3d, Ba 3d, Nb 3d, and O 1s species in CdS/Ba5Nb4O15-MoS2 nanocomposite.
clearly shows the presence of Cd 3d, S 2p, Mo 3d, Ba 3D, Nb 3D, and O 1s peaks of the elements in the CdS@Ba5Nb4O15/MoS2 nanocomposite. The oxidation states of these elements were confirmed from the high-resolution XPS spectra. Fig. 4(c) shows the narrow-scan XPS spectrum of Cd 3d, with peaks at binding energies of 404.96 and 411.73 eV, corresponding to Cd3d5/2 and Cd3d3/2 transitions, respectively [35]. The data confirm that Cd is in the di-positive oxidation state in CdS. Fig. 4(d) shows the narrow-scan XPS spectrum with S2p peaks at binding energies of 161.1 and 162.31 eV [35]. The high-resolution Mo 3d spectrum in Fig. 4(e) shows two peaks at 232.1 and 228.0 eV, which are attributed to the Mo 3d3/2 and Mo 3d5/2 states and confirmed that Mo4+ was the dominant oxidation state. Further, the small peak at the binding energy of 236 eV
was ascribed to the presence of a small amount of Mo6+ [36]. The high-resolution Ba 3d spectrum presented in Fig. 4(f) reveals the divalent oxidation state of Ba, indicated by peaks at 780.6 and 795.5 eV, which are assigned to Ba 3d3/2 and Ba 3d5/2, respectively [37]. The narrow-scan Nb 3d XPS spectrum shows peaks at binding energies of 206.43 and 209.72 eV, which correspond to Nb 3d3/2 and Nb 3d5/2, respectively (Fig. 4(g)) [38]. The O 1s narrow-scan spectrum of the CdS@Ba5Nb4O15/MoS2 composite shown in Fig. 4 (h) could be deconvoluted into three peaks at binding energies of 530.69, 531.94, and 533.3 eV, corresponding to lattice oxygen (OL), defect/vacancy oxygen (OV), and chemisorbed oxygen (OC), respectively [39]. The XPS results unambiguously confirm successful formation of the CdS@Ba5Nb4O15/MoS2 nanocomposites. The
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Fig. 5. (a) DRS, (b) PL, (c) photocurrent, and (d) impedance spectra of CdS, CdS/MoS2, CdS/Ba5Nb4O15, and CdS/Ba5Nb4O15/MoS2 nanostructures. Mott-Schottky plots of (e) CdS nanorods and (f) BNO-MoS2 nanostructures.
optical band-gap and light-harvesting capability of CdS and the nanocomposites were investigated by UV vis diffuse reflectance spectroscopy (Fig. 5(a)). The absorption maximum of bare CdS was located around 540 nm, and the compound was thus capable of harvesting visible light. The addition of MoS2 and Ba5Nb4O15/ MoS2 increased the absorption intensity of CdS in the 550– 800 nm range, which is beneficial for improving the quantum efficiency of the CdS@Ba5Nb4O15/MoS2 composites. The estimated band-gap values for CdS and the nanocomposites were in the range of 2.41–2.415 eV. To understand the mechanism of recombination of the photogenerated charge carriers and the contribution of MoS2 and Ba5Nb4O15 to promoting transfer of the photo-generated electrons and charge separation, the photoluminescence (PL) emission was analyzed as shown in Fig. 5(b). The as-synthesized CdS nanostructures showed a strong emission band with a maximum around 550 nm, ascribed to the surface defects and vacancies [40]. Whereas the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/ MoS2 nanocomposites exhibited peaks at similar positions, the intensity of the composite peaks was much lower than that of
the bare CdS nanostructures. In particular, the CdS@Ba5Nb4O15/ MoS2 nanocomposite showed the lowest PL intensity relative to the other synthesized nanocomposites, indicating that Ba5Nb4O15/ MoS2 promotes transfer of the photo-generated electrons and charge separation in CdS, thereby enhancing the electron-hole pair separation. To further confirm the enhanced charge carrier transport and separation efficiency, the transient photocurrents and impedance of CdS and the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/ MoS2 nanocomposites were photo-electrochemically examined under simulated solar irradiation. Fig. 5(c) shows the photocurrent responses of bare CdS and the CdS/MoS2, CdS@Ba5Nb4O15, and CdS@Ba5Nb4O15/MoS2 nanocomposites with 30 s on/off irradiation cycles. As anticipated, the CdS@Ba5Nb4O15/MoS2 nanocomposites showed the highest photocurrent intensity with respect to time compared with the bare CdS nanostructures and corresponding nanocomposites, owing to the suitable band structure for tunneling photo-generated electrons and holes and higher charge transfer and separation of the former. Moreover, the photocurrent responses with respect to time were highly reproducible for sev-
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Fig. 6. (a) Amount of hydrogen evolved in 15 mL of aqueous solution containing lactic acid (3 mL) and DI water (12 mL), with suspended CdS/Ba5Nb4O15/MoS2 nanostructures (1 mg) containing 0, 1, 3, 5, 6, 7, and 10 wt% Ba5Nb4O15/MoS2 nanostructures. (b) Comparison of the optimized photocatalyst with other catalysts (1: CdS, 2: B-BNO/CdS, 3: UT-BNO/CdS, 4: CdS/Pt, 5: CdS/B-MoS2, 6: UT-MoS2, and 7: UT-MoS2-BNO/CdS). (c) Effect of photocatalyst loading on hydrogen production rate (photocatalyst: CdS/WS2MoS2 (6 wt%) nanocomposite; photocatalyst loading: 1, 3, 5, and 7 mg; time: 5 h; water/lactic acid: 12:3 mL). (d) Effect of concentration of sacrificial lactic acid on the hydrogen evolution rate (photocatalyst: CdS/Ba5Nb4O15/MoS2 nanocomposite; photocatalyst dose: 1 mg; time: 5 h). (e) Hydrogen production in recycling experiments using the optimized CdS/Ba5Nb4O15-MoS2 and CdS/MoS2 nanocomposites. (f) Long-term stability of the optimized CdS/Ba5Nb4O15-MoS2 nanocomposites.
eral on/off cycles and remained stable, demonstrating that the presence of MoS2 and Ba5Nb4O15 on the CdS nanorods can effectively prevent photocorrosion of CdS [41]. Furthermore, electrochemical impedance spectral (EIS) analysis (Fig. 5(d)) showed a smaller semicircular region for the CdS@Ba5Nb4O15/MoS2 nanocomposite than the other nanocomposites and bare CdS nanostructures, demonstrating that CdS@Ba5Nb4O15/MoS2 exhibited the fastest interfacial charge transfer due to the MoS2 and Ba5Nb4O15 co-catalysts [42]. Based on the above experimental data, it is apparent that embedding MoS2 and Ba5Nb4O15 on CdS can effectively enhance the rate of photocatalytic hydrogen evolution by the CdS nanostructures. In order to evaluate the feasibility of transfer of the photogenerated electrons from the CdS nanostructures to the Ba5Nb4O15/ MoS2 nanocomposites, the flat-band potential (VFB) was estimated by using Mott Schottky plots at a frequency of 1 kHz. The estimated conduction band potentials of the CdS and Ba5Nb4O15/
MoS2 nanocomposites were 1.12 and 0.74 V vs. Ag/AgCl, respectively (Fig. 5(e, f)). The results demonstrated that the estimated conduction band potential of Ba5Nb4O15/MoS2 is more positive than that of the CdS nanostructures and more negative than that of the H+/H2 redox couple ( 0.41 V vs. NHE at pH = 7) [43]. Hence, Ba5Nb4O15/MoS2 served as a co-catalyst and contributed to efficient solar-driven hydrogen evolution. The photocatalytic activity of hydrogen was evaluated by using lactic acid as a hole scavenger under simulated solar irradiation. Here, we used an air-tight reaction quartz beaker that was externally irradiated with a solar simulator. The evolved hydrogen was sampled with a gas-tight syringe and analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector. As depicted in Fig. 6(a), the as-synthesized bare CdS nanostructures generated a very low amount of hydrogen (2.54 mmol h–1 g– 1 ) due to the fast recombination of the electron–hole pairs [44]. For comparison bare Ba5Nb4O15 and Ba5Nb4O15/MoS2 nanocatalysts
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also tested for hydrogen evolution and we noticed very little hydrogen was generated using pure MoS2 (0.04 mmol h 1 g 1) and Ba5Nb4O15/MoS2 (0.58 mmol h 1 g 1) nanostructures, indicating that bare Ba5Nb4O15 and Ba5Nb4O15/MoS2 nanostructures are not efficient for photocatalytic hydrogen evolution. The CdS nanocrystals were incorporated with MoS2/Ba5Nb4O15 and the loading of MoS2/Ba5Nb4O15 on CdS was optimized to maximize the hydrogen production. Notably, the presence of a small amount of MoS2/Ba5Nb4O15 on CdS led to greater hydrogen production and increased with increasing loading of MoS2/Ba5Nb4O15 on CdS up to the optimum level of 6.0 wt%, with a decrease at higher loadings (Fig. 6(a)). The reduced rate of hydrogen production at high loading may be due to the increased coverage of the CdS surface by MoS2/ Ba5Nb4O15, which reduces the area of CdS exposed to light and consequently lowers the rate of electron-hole pair generation [45]. The photocatalyst loading and hole scavenger concentration were also optimized; 1 mg of catalyst and 20 vol% of hole scavenger were suitable for high hydrogen production (Fig. 6(c, d)). Under the optimized conditions, the maximum rate of hydrogen evolution was 147 mmolh 1g 1, achieved using CdS@MoS2/Ba5Nb4O15 (UTMoS2-BNO/CdS). This rate is significantly higher than that achieved with CdS, CdS@ Ba5Nb4O15 (B-BNO/CdS), ultrathin CdS@Ba5Nb4O15 (UT-BNO/CdS), MoS2/CdS (B-MoS2/CdS), ultrathin MoS2/CdS (UTMoS2/CdS) and CdS/Pt (Fig. 6(b)). Furthermore, the observed rate of hydrogen evolution in the presence of CdS@MoS2/Ba5Nb4O15 was much faster than that achieved with several cocatalystmodified CdS nanostructures reported earlier (Table S1). In addition to the rate of photocatalytic of hydrogen evolution, the apparent quantum efficiency (QEs) is another important parameter for assessing the photocatalytic activity. The optimized CdS@MoS2/ Ba5Nb4O15 nanocomposite was evaluated by visible-light irradiation using a 150 W Xe lamp with a 425 nm band-pass filter, and the quantum yield was estimated to be around 28.2%. Compared to the photocatalytic activity, the catalyst stability is more important in terms of practical applications. In order to verify the stability of the optimized CdS@MoS2/Ba5Nb4O15 nanostructures, recycling experiments were carried out as shown in in Fig. 6(e). From Fig. 6(e), it is clear that almost identical amounts of hydrogen were produced in all five recycling experiments. However, there was a minor decrease of the amount of hydrogen generated in the second and third cycles compared to the first cycle, which is attributed to the decreased concentration of lactic acid due to its conversion to pyruvic acid during the reaction. To verify this speculation, an additional 3 mL of lactic acid solution was added, and the hydrogen evolution rate in the fourth and fifth cycle was similar to that in the first cycle, indicating that the catalyst was stable over five cycles. Furthermore, we have noticed that the CdS@MoS2/Ba5Nb4O15 nanostructures are more stable than the CdS/MoS2 nanostructures. Furthermore, the CdS@MoS2/Ba5Nb4O15 nanostructures showed remarkable long-term photostability up to 40 h (Fig. 6(f)), indicating that the nanocomposite is a stable, noble-metal-free catalyst for hydrogen production. The enhancement of the photocatalytic hydrogen evolution performance and the extraordinary stability of the as-synthesized CdS@MoS2/Ba5Nb4O15 nanocomposites can be attributed to the efficient MoS2/Ba5Nb4O15 co-catalyst system, having lightharvesting capability and suitable reduction potentials with rich active sites. Based on the DRS, PL, photocurrent, and impedance analyses, it is concluded that the enhanced rate of photocatalytic hydrogen evolution in the presence of the CdS@MoS2/Ba5Nb4O15 nanocomposites relative to the bare CdS nanostructures can be attributed to the following: (i) the synthesized CdS@MoS2/Ba5Nb4O15 nanocomposites have more prominent sunlight-harvesting capability than the as-synthesized CdS nanostructures. These peculiar properties may facilitate the production of photogenerated electrons from the nanocomposite, leading to enhancement
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Scheme 2. Schematic illustration of the proposed mechanism for utilization of the CdS/Ba5Nb4O15-MoS2 nanocomposites as photocatalysts for hydrogen production under simulated solar irradiation using lactic acid as a sacrificial agent.
of the photocatalytic hydrogen evolution rate. (ii) The strong, intimate contact between Ba5Nb4O15, MoS2, and the CdS nanostructures contributes to much more efficient transfer of the photogenerated electrons between adjacent nanostructures, providing more active reaction sites for efficient hydrogen production. (iii) The suitable band-edge potentials of CdS and Ba5Nb4O15-MoS2 enhance the reduction ability of the photo-excited electrons and are more favorable for photocatalytic hydrogen generation [46– 49]. A schematic summary of the proposed mechanism for utilization of the CdS/Ba5Nb4O15-MoS2 nanocomposites as photocatalysts for hydrogen production under simulated solar irradiation using lactic acid as a sacrificial agent is presented in Scheme 2. 3. Conclusions Hybrid nanostructures combining perovskite (Ba5Nb4O15)-MoS2 ultrathin nanosheets embedded with CdS nanorods were prepared via a simple hydrothermal method using an ultrasonicationassisted chemical process. The resulting nanostructures were characterized by strong, intimate contact between Ba5Nb4O15, MoS2, and the CdS nanostructures and exhibited accessible charge transfer with high visible-light harvesting capability. The designed nano-hybrid nanostructures gave rise to a high hydrogen production rate of 147 mmolg–1h–1 in the presence of lactic acid as a sacrificial electron donor under simulated solar irradiation. This value is much higher than those of the CdS/MoS2 and CdS/Ba5Nb4O15 nanostructures and that of the expensive CdS/Pt benchmark catalyst. Furthermore, the rate of solar-driven hydrogen evolution in the presence of the ultrathin perovskite Ba5Nb4O15/MoS2 nanohybrid on the CdS nanorods is much faster than that of several noble-metal-free co-catalyst-modified CdS nanostructures reported earlier. The proposed designed strategy should pave the way for developing other highly active noble-metal-free photocatalysts for water splitting applications. Acknowledgements This work was financially supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (2014R1A4A1001690 and 2016R1E1A1A01941978). This research was also supported in part by the Max Planck POSTECH/KOREA Research Initiative Program [Grant No. 2016 K1A4A4A01922028] through the MEST’s NRF funding.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2017.06.033.
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