- Email: [email protected]
Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation
Journal Pre-proofs Full Length Article Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation A. Putta Rangappa, D. Praveen Kumar, Madhusudana Gopannagari, D. Amaranatha Reddy, Yul Hong, Yujin Kim, Tae Kyu Kim PII: DOI: Reference:
S0169-4332(19)33619-0 https://doi.org/10.1016/j.apsusc.2019.144803 APSUSC 144803
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
16 August 2019 18 November 2019 19 November 2019
Please cite this article as: A. Putta Rangappa, D. Praveen Kumar, M. Gopannagari, D. Amaranatha Reddy, Y. Hong, Y. Kim, T. Kyu Kim, Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.144803
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation A. Putta Rangappa1, D. Praveen Kumar1, Madhusudana Gopannagari, D. Amaranatha Reddy, Yul Hong, Yujin Kim, and Tae Kyu Kim* Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea
Abstract The development of low-cost and noble-metal-free catalysts for the photoconversion of water into hydrogen (H2) is of great interest. Here, 2D tin(IV) sulfide (SnS2) ultrathin nanosheets as coǦ catalysts are coupled with 1D cadmium sulfide (CdS) nanorods for the photosplitting of water into H2. The design of the catalyst can facilitate the passivation of the physiochemical properties of CdS and enhance H2 evolution activity. The prepared CdS/SnS2 composite catalyst increases the H2 generation activity and exhibits excellent and continuous long-term photostability. The H2 evolution rate of the optimized CdS/SnS2 composite is approximately 9-fold that of pristine CdS nanorods. The characterization results of the CdS/SnS2 composite reveal that the loading of SnS2 can enhance the synergistic effects of the photocatalyst due to the effective separation, large number of exposed catalytic sites, and highly dispersed nature of the layered SnS 2. Several characterization outcomes of CdS/SnS2 are examined in detail (e.g., structural and surface elemental results of transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy). Further, the optoelectrical properties and charge-carrier excitations
are investigated via ultraviolet diffuse reflectance spectroscopy, photoluminescence spectroscopy, and photoelectrochemical analysis. The proposed CdS/SnS2 composite is a promising low-cost, noble-metal-free, and high-efficiency catalyst for the photocatalytic water-reduction reaction. Keywords: SnS2 ultrathin nanosheets, 1D/2D system, hydrogen production, water splitting, charge-transfer. *Corresponding author. E-mail address: [email protected] (T.K. Kim) 1
The author contributed equally to this work.
1. Introduction The energy demand is predicted to double over the next 30 years, and at present, the main sources of energy production are fossil fuels. However, continued dependence on fossil fuels to satisfy energy requirements will lead to significant global warming and environmental pollution. Additionally, fossil fuel resources are rapidly being depleted; therefore, it necessary to satisfy the growing energy requirements in a sustainable way, without harming the environment. Solar energy is a continuous and abundant source but is presently not storable [1–4]. Hence, there is an urgent need to develop an approach for storing solar energy. The photosplitting of water into hydrogen (H2) and oxygen (O2) utilizing solar energy is considered one of the effective ways to preserve solar light. The generation of H2 from solar-driven water reduction is an efficient, clean, and sustainable approach. Photocatalytic reactions mediated by semiconductors can harvest solar energy and convert it into chemical energy. These processes can be utilized in various important environmental chemistry applications [5–7].
Several semiconductor materials have been explored for photocatalytic water splitting reactions. Among them, oxide-based materials with wide bandgaps have limitations in utilizing a broad spectrum; they only absorb light in the ultraviolet (UV) region of the solar spectrum [8]. Nitrides have poor aqueous stability [9]. The synthesis of phosphides involves a high temperature and the flammable element phosphorus, and phosphides have limited applications. Sulfide (S)based semiconductors are interesting owing to their distinct optoelectronic properties [10]. It is highly desirable to select materials with suitable bandgaps and band potentials for achieving high photocatalytic efficiencies for H2 production via the effective absorption of visible light. In this regard, cadmium sulfide (CdS) is one of the well-known S-based visible-light active semiconductor photocatalysts, and it is easy to prepare in nanostructures [11–14]. Various nanostructures of CdS have been used in diverse applications, such as sensors, photovoltaic cells, solar cells, photoluminescent strategies, and electro-optic modulators [15–17]. Different CdSbased nanostructures have been developed with valence- and conduction-band edges tuned for applications in diverse fields. However, the utilization of 2D films and 0D nanoparticles is limited in photocatalysis owing to the lack of continuous conducting pathways. It is believed that 1D nanostructures have better charge-transport properties [18–20]. CdS nanorods have great potential as catalysts, and they have attracted considerable attention for photocatalytic H2 evolution because of their excellent water-reduction potential and high charge-separation efficiency on the surface [21,22]. However, the CdS semiconductor is not highly active for wavelengths above 520 nm, limiting the utilization of solar light. Additionally, the fast charge-carrier recombination and photocorrosion reduce the efficiency of bare CdS for photocatalytic applications [23–25]. Thus, researchers have attempted to co-sensitize CdS with different suitable co-catalysts (e.g., CdSe,
CdTe, PbS, Ag2S, ZnO, Co3(PO4)2) to attain high photocatalytic performance [26–31]. Recently, 2D co-catalyzed CdS has been widely investigated owing to its peculiar properties [32–37]. Among the materials, 2D materials are the most popular, and they exhibit fascinating properties. Promising alternative co-catalysts include 2D transition-metal dichalcogenides (TMDs), which exhibit distinctive properties, such as direct bandgaps, large atomic thickness, strong spin–orbit coupling, and advantageous electronic and mechanical characteristics, making them interesting for basic energy conversion research. Among the various semiconducting TMDs, such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), and tin(IV) sulfide (SnS2), MoS2 and WS2 are important TMD semiconductor materials owing to their attractive electrical properties, and thus, they have been widely investigated. There is a need to develop highly efficient, stable photocatalysts. However, the poor charge transfer, low stability, and hydrophobic characteristics of MoS2 [38–44] and the low density and reactivity of active sites and limited corrosion stability of WS2 are major disadvantages for photocatalytic reactions [45]. The TMD SnS 2 (IV–VI metal sulfide semiconductor) with bandgap 2.0–2.5 eV [46] is widely applicable in diverse fields (e.g., photocatalysis, solar cells, sensors and batteries) due to its excellent properties, such as its high optical absorption coefficient, tunable bandgap, and natural abundance [47–50]. SnS2 can be an excellent alternative for photocatalytic applications owing to its good stability in both acidic and alkaline solutions, high thermal stability, and nontoxicity [51,52]. The exceptional range of visiblelight absorption of SnS2 allows the construction of heterojunctions with SnS2 for photocatalytic water splitting into H2. Different types of heterostructures have been reported with SnS 2 as a cocatalyst, confirming that the formation of heterostructures with SnS2 for effective charge separation is beneficial for photocatalytic applications [53, 54]. Photocatalytic systems with the direct Z-scheme process for enhanced photocatalytic activity have been reported [55–59]. The
CdS/SnS2 nanohybrid, which facilitates electron/hole transport, suppresses the charge recombination of CdS and promotes photocatalytic water splitting H2 evolution. In this study, we developed a low-cost, highly efficient, and stable semiconductor CdS/SnS2 nanohybrid via facile synthesis techniques and employed it for photocatalytic water splitting H2 evolution. This nanohybrid structure exhibited a robust H2 production rate of 20.2 (mmol h−1 g−1) while under photoirradiation for 3 h. This activity was significantly better than that of bare SnS 2 and 9-fold higher than that of pristine CdS. More importantly, the CdS/SnS2 nanohybrid catalyst exhibited long-term stability over 30 h with continuous photoirradiation, indicating its potential for long-term practical applications. The plausible mechanism of CdS/SnS2 for photocatalytic H2 generation was elucidated along with the effective charge separation based on the band alignment between CdS and SnS2. This study provides insights regarding the development of inexpensive and noble-metal-free semiconductor photocatalysts for the H2 evolution reaction system under solar light.
2. Experimental 2.1. Raw Materials Thioacetamide-TAA
(C2H5SN),
tin
(IV)
chloride
pentahydrate
(SnCl4∙5H2O),
polyvinylpyrrolidone (PVP; molecular weight: 30000 g·mol−1), cadmium acetate dihydrate (Cd(AC)2∙2H2O), ethylene glycol (EG, (C2H6O2)), ethylenediamine anhydrous (EDA, C2H8N2), thiourea (CH4N2S), ethanol, N,N-dimethylformamide (DMF), lactic acid (LA), and deionized (DI) water were used. 2.2. Preparation of CdS Nanorods
In the preparation process, 20.25 mmol of Cd(AC)2∙2H2O and 60.75 mmol of CH4N2S were suspended in 60 mL of EDA and ultrasonicated for 1 h to ensure a consistent dispersion. This dispersion was transferred to a clean autoclave (Teflon vessel) and heated to 160°C for 48 h. Then, the reaction mass was cooled to 25–27°C. The resulting yellow pasty mass was collected and cleaned thoroughly with DI water and then ethanol. Finally, the obtained material was dried in a hot air oven at 80°C for 10–12 h. 2.3. Preparation of SnS2 Ultrathin Nanosheets In the synthesis process, SnCl4∙5H2O and C2H5SN with a mass ratio of 1:2, as well as 2 g of PVP, were suspended in 50 mL of EG. The suspension was vigorously stirred for 30 min to obtain a homogeneous dispersion. Then, the mixture was transferred to an autoclave and treated hydrothermally at 160°C for 12 h. After the completion of the reactions, the obtained product was cleaned thoroughly with DI water and then ethanol and dried in a hot air oven at 80°C for 10–12 h. 2.4. Preparation of CdS/SnS2 Nanohybrid Structure The CdS/SnS2 nanohybrid structure was prepared via the solvent exfoliation method in DMF. In this process, the required amount of bulk SnS2 was first dispersed in the solvent (10 mL of DMF) and sonicated for 2 h to achieve exfoliation from the bulk SnS2. CdS nanorods were added, and the sonication was continued for 1 h. The well-dispersed products were stirred for 12 h at room temperature to deposit SnS2 ultrathin nanosheets onto the CdS nanorods. The obtained products were cleaned with DI water followed by ethanol several times until the solvent was removed completely. Then, the products were dried at 80Ԩ for 10–12 h. The overall synthesis procedures for SnS2, CdS, and CdS/SnS2 nanohybrids are illustrated in Scheme 1.
2.5. Characterization Details of Synthesized Products The photocatalyst powder was characterized as follows. X-ray diffraction (XRD) patterns were collected using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed using Al Kα radiation (1486.6 eV) with an energy of 15 kV/150 W. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV. Diffuse reflectance spectra were recorded using a UV-1800 SHIMADZU, and photoluminescence (PL) spectra were measured using a Hitachi F-7000 florescence spectrometer. Photoelectrochemical (PEC) experiments were performed using a CHI 617B unit with a threeelectrode cell. Platinum wire was used as a counter electrode, and silver/silver chloride (Ag/AgCl) was used as a reference electrode. A solar simulator with an AM 1.5G filter and a 150-W xenon (Xe) lamp (ABET Technologies) was used as the light source. The silicon (Si) reference cell was used to adjust the light intensity to 1 sun (100 W/m2). A 0.5 M solution of Na2SO4 was used as the electrolyte. The working electrode was prepared by cleaning an In-doped SnO2 (ITO)-coated glass with ethanol. The photocatalyst (10 mg) was dispersed in ethanol (450 μL) and Nafion (50 μL) via soft ultrasonication, and the uniform suspension (30 μL) was added dropwise directly onto the cleaned ITO glass substrate using a micro-syringe. The substrate and suspension were placed in an oven at 100°C for 3 h to accelerate the drying. The solar simulator was used for the measurement of photoresponses through on–off cycling. Mott–Schottky (MS) measurements were performed under dark conditions at a frequency of 1 kHz using an impedance spectra analyzer with a fixed potentiostat. The other conditions were identical to those used for the PEC experiments. The
obtained potentials vs. Ag/AgCl were transformed to the normal hydrogen electrode (NHE) scale using the equation ENHE = EAg/AgCl + 0.197 (V) at a pH of 7. 2.6. Photocatalytic Assessments Photocatalytic experiments to generate H2 were performed in a quartz reactor (150 mL) containing a solution of 15 mL of water, 20 vol. % hole scavenger LA, and 1 mg of synthesized material. The reactor was closed tightly with an airtight rubber septum after all the materials were added. The solar simulator with an AM 1.5G filter and a 150-W Xe lamp (ABET Technologies) was used as the light source. The Si reference cell was used to adjust the light intensity to 1 sun (100 W/m 2). Before commencing irradiation, the system was evacuated for 30 min and degasified with Ar for 30 min to nullify the presence of air in the system. The amount of H2 produced was evaluated by an offline gas chromatograph (Young Lin Autochro-3000) equipped with a thermal conductivity detector.
3. Results and Discussion 3.1. Structural Analyses The crystal structures and phases of the as-synthesized CdS, SnS2, and CdS/SnS2 were analyzed via powder XRD. Fig. S1 in Supplementary Data (SD) shows the diffraction peaks of CdS, SnS 2, and the CdS/SnS2 nanohybrid structure. The as-prepared CdS exhibited a hexagonal lattice structure and strongly matched JCPDS Card #89-2944. No peaks other than that corresponding to CdS were observed. The diffraction peaks of SnS2 confirmed the existence of a hexagonal lattice structure. The peaks at 28.22°, 32.11°, 52.45°, 59.55°, and 70.35° corresponded to the planes of (100), (101), (111), (112), and (113), respectively, closely matching the structure of SnS2 (JCPDS
Card # 40-1467). The high-intensity peak at 52.45° was attributed to the (101) plane of the hexagonal SnS2 nanosheets, and the diffraction peaks corresponding to the (100) and (101) planes had high intensities compared with the standard values. The results indicated the preferential orientation of the (101) plane. This distinct orientation of planes resulted from the capping of PVP. SnS2 nanostructures synthesized with appropriately exposed facets can enhance visible-lightsensitive photocatalysts [60]. The diffraction peaks of SnS2 were undetectable for the composite, indicating the relatively low content of SnS2 products in the CdS/SnS2 nanohybrid structure. Moreover, we noticed a slight increase in the diffraction intensity of CdS/SnS2 compared with bare CdS, probably due to the higher atomic scattering factor. 3.2 Morphology and Surface Elemental Analysis The chemical states and elemental composition of the optimized CdS/SnS2 nanocomposite were examined via XPS, as shown in Fig. 1. The XPS spectra clearly indicate the presence of Sn, Cd, and S in the composite. Additionally, the elements and their chemical states were confirmed by the high-resolution Cd 3d, Sn 3d, and S 2p X-ray photoelectron spectra. Fig. 1(a) shows the complete survey spectrum, and Figs. 1(b)–(d) show the high-resolution S 2p, Sn 3d, and Cd 3d XPS spectra of the CdS/SnS2 nanohybrid structure, respectively. The peaks of the Cd 3d spectrum were observed at binding energies 405.1 and 411.9 eV, corresponding to Cd 3d5/2 and Cd 3d3/2, respectively. The peaks of Sn 3d and S 2p appeared at 486.7 eV (Sn 3d5/2) and 495.0 eV (Sn 3d3/2) and at 161.2 eV (2 p3/2) and 162.4 eV (2 p1/2), respectively. When CdS was coupled with SnS2, the binding energy of Cd 3d was blue-shifted (Fig. S4), which is an indication of the strong interactions between CdS and SnS2. Similarly, Zhang et al. also reported the Cd 3d XPS spectrum shift for the CdS/WS2 nanocomposite [61].
The TEM image of pure SnS2 clearly confirms the nanosheets’ morphology and image, as displayed in Fig. S3. The morphologies of the samples were analyzed via TEM, as shown in Fig. 2. The morphology of the composite clearly indicated that the ultrathin SnS2 sheets were uniformly deposited onto the CdS nanorods. The exfoliated SnS2 sheets covered most of the CdS nanorods, increasing the amount of exposed active sites for photocatalytic applications. The HRTEM images of the optimized CdS/SnS2 (Fig. 2(b)) clearly show the presence of the ultrathin nanosheets of SnS2 on the CdS nanorods. Additionally, the elemental composition of the optimized hybrid catalyst was analyzed via energy-dispersive X-ray spectroscopy. As shown in Fig. 2(c), the CdS/SnS2 composite exhibited homogeneous distributions of Cd and S, as well as a relatively weak Sn signal, which mostly appeared on the surfaces of the CdS nanorods. In addition, the HRTEM image of CdS/SnS2 composite clearly confirms the presence of a few layered SnS2 nanosheets on the CdS nanorods by the well-matched lattice fringes of individual materials. However, we verified the presence of the few layered SnS2 nanosheets on the CdS nanorods by EDS mapping, and the results are displayed in Figs. 2(d)–(f). All these findings indicate the successful synthesis of the CdS/SnS2 nanocomposite, where the CdS nanorods were covered by the ultrathin SnS2 nanosheets. 3.3. Optical Studies The optical absorption of CdS and the composite products was investigated via UV diffuse reflectance spectroscopy (UV-DRS). As shown in Fig. 3(a), the CdS and hybrid CdS/SnS2 samples exhibited the classical semiconductor light-absorption feature. In the optical range of >520 nm, the CdS samples with SnS2 exhibited significantly more light absorption compared with CdS. The bandgaps of CdS and composite were calculated to be 2.3 eV. However, compositing CdS with SnS2 may not alter the inherent band structure of the CdS semiconductor; instead, the strong
interaction between the materials leads to increased photoabsorption. The increased visible-light absorption significantly contributed to the harvesting of photons, which typically enhances the H2 generation. UV-DRS was further analyzed for pristine SnS2 (results displayed in Fig. S5), and its bandgap was then calculated (2.08 eV). The charge-carrier transfer and recombination capability of the CdS and composite materials were examined via PL spectroscopy, as shown in Fig. 3(b). The spectra were obtained with an incident excitation wavelength of 380 nm. A strong emission peak around 550 nm was observed for both CdS and CdS/SnS2, owing to the band edge emission property of the CdS nanorods. The reduced peak intensity of the composite indicates the effective transfer and separation of photoinduced charge carriers [62–64]. Therefore, the sensitization of CdS with SnS2 accelerated the excited electron transfer onto the surface, with excellent separation of charge carriers on the composite, and enhanced the photocatalytic H2 production. A PEC analysis was performed to investigate the charge separation on CdS and the composite. The spectra shown in Fig. 3(c) were obtained using an electrochemical potentiostat with working electrodes coated with CdS and CdS/SnS2 nanohybrids on an ITO glass substrate. As shown in Fig. 3(c), the photocurrent intensity of each spectrum increased when the simulated light was turned on, owing to the electrons induced by the light. Additionally, as expected, the CdS/SnS2 nanohybrid exhibited a higher intensity than bare CdS. This enhanced photocurrent indicates the good separation of migrated charge carriers on CdS/SnS2. All the electrons moved to the surface of the catalyst and participated in the reduction of H+ to H2, yielding a large amount of H2. Electrochemical impedance analysis of the CdS nanorods and CdS/SnS2 nanohybrid was performed, as shown in Fig. 3(d). The small semicircular region was smaller for the CdS/SnS 2
nanohybrid than for the bare CdS nanorods, indicating the faster interfacial charge transfer of the CdS/SnS2 nanohybrid. Together, the experimental results suggest that the CdS/SnS2 nanohybrids had enhanced catalytic activity compared with the bare CdS nanostructures under solar light. The observed features strongly indicate that the SnS2 nanosheets greatly influenced the H2 evolution rate of the CdS nanorods under solar light irradiation. 3.4. MS Analysis To elucidate the mechanism underlying the photocatalytic H2 production, an MS analysis was performed to evaluate the exact potentials (conduction band (CB)) of the ultrasonicated CdS nanorods, SnS2 nanosheets, and CdS/SnS2 nanohybrid structures, as shown in Figs. S2(a)–(e) of SD. The band potentials of CdS/SnS2, CdS, and SnS2 were estimated to be -0.958, -1.195, and 0.81 V (vs. EAg/AgCl), respectively. The obtained potentials vs. Ag/AgCl were transformed to the NHE scale using the equation ENHE = EAg/AgCl + 0.197 (V) at pH = 7 [65]. The CB potentials (vs. ENHE) were calculated to be -0.761, -0.998, and -0.613 V for CdS/SnS2, CdS, and SnS2, respectively. The CB potential of CdS/SnS2 was between those of CdS and SnS2. It was more negative than the H+ reduction potential (–0.410 V vs. ENHE at a pH of 7) and more positive than the CB potential of bare CdS nanorods, indicating the efficient utilization of photogenerated charge carriers, which enhanced the photocatalytic H2 production. 3.5. Photocatalytic Activity The photocatalytic activity of CdS/SnS2 for H2 generation was evaluated in the presence of LA under solar light. Fig. 4(a) shows the weight percentage (wt.%) effect of SnS 2 on CdS regarding the photocatalytic H2 performance. The SnS2 loading amount of 5 wt.% yielded the highest H2 production rate (20.2 mmol h−1 g−1), which was enhanced by a factor of 9.2 compared with that of
pristine CdS. Lower activity was observed for larger amounts of SnS2 loading (>5 wt.%), owing to the minimized active sites of CdS with fully covered SnS2 nanosheets. The obtained results were compared with previous reports by providing a comparison table for H2 evolution rates of different CdS-based composites, and they are displayed in Table S1. The reproducibility assessment shown in Fig. 4(b) indicates the high consistency of the H2 evolution rate of the CdS/SnS2 hybrid catalyst. The photocatalytic activity catalyzed by 5 wt.% CdS/SnS2 catalyst in H2 generation was investigated, as shown in Fig. 4(c). Different amounts of catalyst ranging from 1 to 6 mg were used to perform the experiments under identical conditions. As the amount of photocatalyst (5 wt.% CdS/SnS2) increased from 1 to 4 mg in the photocatalytic system (20% LA in 15 mL of reaction solution), the H2 production rate increased to 34.7 mmol h−1, as shown in Fig. 4(c). Saturation of the H2 evolution was observed for the system in which the catalyst amount was >4 mg, because an excessive amount of catalyst in the system shielded the dispersed photocatalyst particles. Additionally, the effect of the scavenger (LA) volume on the H2 generation rate was evaluated for the optimized CdS/SnS2 photocatalyst. Different experiments were conducted in which the LA volume was varied from 0 to 5 mL in 15 mL of the reaction solution. The optimum H 2 evolution rate of 20.2 mmol h−1 g−1 was achieved with the photocatalytic system comprising 3 mL of LA. A further increase in the LA concentration reduced the H2 generation rate, possibly because the presence of an excessive amount of scavenger (i.e., >3 mL) in the system led to a high rate of intermediate formation. Recycling experiments (Fig. 4(e)) provided the H2 generation rate on CdS/SnS2 for five repeated cycles. Each cycle was conducted under identical conditions, and the rate of H2 production significantly decreased after the third cycle, possibly owing to the reduced scavenger concentration in the photocatalytic system [66–68].
However, when an excessive amount (3 mL) of LA was added to the system, the H 2 generation rate approached that observed in the first cycle. This suggests that the consumption of LA through oxidation to pyruvic acid limited the H2 evolution rate in the photocatalytic reaction [69]. The stability of the optimized CdS/SnS2 nanocomposite catalyst was investigated by continuously measuring the H2 production rate under continuous solar light irradiation. As shown in Fig. 4(f), the CdS/SnS2 photocatalyst exhibited high stability for 30 h, indicating its potential for long-term practical applications. A small decline in the H2 production rate was observed after 30 h, possibly owing to the large number of products deposited on the catalyst, as well as the deficiency of the scavenger in the system, which was converted into pyruvic acid via oxidation. Additionally, the photoreactor may have been filled with a large amount of H2 gas, which was generated by a continuous reaction. With repeated evacuation and Ar purging of the system after 30 h, the H2 generation rate for the next 3 h of the photoreaction remained nearly unchanged and was identical to that observed for the first 3 h of the photoreaction. Additionally, we performed photocatalytic studies for simple ultrasonicated CdS, SnS2, and their simple mixer, and they showed a much lower rate of H2 production than the CdS/SnS2 composite due to the high recombination in pristine materials and very weak interactions in the simple mixing of pristine materials. 3.6. Photocatalytic Mechanism According to the analysis results, a plausible mechanism for water reduction into H2 catalyzed by the CdS/SnS2 photocatalyst was proposed (Scheme 2). The electron–hole pairs were generated on CdS/SnS2 under the illumination of solar light. The UV-DRS results revealed the excellent visiblelight absorption capability of the CdS/SnS2 photocatalyst. According to the CB potential levels of
CdS and SnS2, the photogenerated charge carriers (electrons) of CdS were transferred to the CB of SnS2. The lifetime of the electrons was prolonged while the electrons were transferred from CdS to SnS2 through the interfacial contact between CdS and SnS2. The excited electrons rapidly migrated onto the CB of SnS2, efficiently reducing H+ to produce H2. On the other side, the photogenerated holes in the valence band (VB) of SnS2 moved to the VB of CdS, and the scavenger accepted the holes, transforming into pyruvic acid. Thus, the complete cycle of H 2 generation under solar light was accomplished.
4. Conclusion Layered ultrathin SnS2 sheets are attractive noble-metal-free, highly efficient, and stable cocatalysts for photocatalytic H2 generation from water. A high H2 evolution rate was achieved using a CdS/SnS2 photocatalyst, which was 9 times higher than that of bare CdS. Combining highly visible-light active SnS2 nanosheets as co-catalysts with CdS led to a large number of active sites for catalytic applications and enhanced optical properties compared with bare CdS. The CdS/SnS2 photocatalyst exhibited stability for >30 h, with no degradation in the rate of H2 evolution. The development of low-cost and stable photocatalytic systems has attracted considerable attention for the advancement of highly efficient photocatalysts for long-term practical applications. Hence, the proposed system provides significant motivation for further studies on energy and environmental applications. This approach might provide valuable guidance for the development of metal dichalcogenide (MX2) 2D materials.
Supplementary Data
XRD patterns of CdS, SnS2, and CdS/SnS2 nanohybrids; Mott-Schottky measurements of CdS, SnS2, and CdS/SnS2 nanohybrids; TEM image of ultrathin SnS2 nanosheets; High resolution XPS spectra of Cd 3d of CdS and CdS/SnS2 nanohybrids; UV–visible diffuse reflectance spectrum of SnS2, Comparison table of H2 evolution rate of different CdS and SnS2 based nanocomposites.
Acknowledgements Funding: This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (grant numbers: 2016R1E1A1A01941978 and 2016K1A4A4A01922028).
References [1]
N.S. Lewis, D.G. Nocera, Powering the planet: Chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. 103 (2016) 15729–15735.
[2]
W. Wei, H. Wang, Y.H. Hu, Unusual particle-size-induced promoter-to-poison transition of ZrN in counter electrodes for dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 14350– 14357.
[3]
H. Wang, W. Wei, Y. H. Hu, Efficient ZnO-based counter electrodes for dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 6622–66288.
[4]
H. Wang, Y.H. Hu, Graphene as a counter electrode material for dye-sensitized solar cells, Energy. Environ. Sci. 5 (2012) 8182–8188.
[5]
C.M. Magdalane, K. Kaviyarasu, N. Matinise, N. Mayedwa, N. Mongwaketsi, D. Letsholathebe, G.T. Mola, N.A. Dhabi, M.V. Arasu, M. Henini, J. Kennedy, M. Maaza, B. Jeyaraj, Evaluation on La2O3 garlanded ceria heterostructured binary metal oxide nanoplates for UV/visible light induced removal of organic dye from urban wastewater, S. African J. Chem. Eng. 26 (2018) 49–60.
[6]
M. Sivachidambaram, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, H.A. Al-Lohedan, R.J. Ramalingam, A novel synthesis protocol for Co3O4 nanocatalysts and their catalytic applications, RSC. Adv. 7 (2017) 38861–38870
[7]
Y.S. Reddy, C.M. Magdalane, K. Kaviyarasu, G.T. Mola, J. Kennedy, M. Maaza, Equilibrium and kinetic studies of the adsorption of acid blue 9 and Safranin O from aqueous solutions by MgO decked FLG coated Fuller's earth, J. Phys. Chem. Solids 123 (2018) 43– 51.
[8]
K. Maeda, K. Domen, New non-oxide photocatalysts designed for overall water splitting under visible light, J. Phys. Chem C 111 (2017) 7851–7861.
[9]
Y. Wu, P. Lazic, G. Hautier, K. Persson, G. Ceder, First principles high throughput screening of oxynitrides for water-splitting photocatalysts, Energy Environ. Sci. 6 (2013) 157–168.
[10] D.V. Shinde, S.A. Patil, K. Cho, D.Y. Ahn, N. K. Shrestha, R. S. Mane, S. H. Han, Revisiting metal sulfide semiconductors: A solutionǦ based general protocol for thin film formation, hall effect measurement, and application prospects, Adv. Funct. Mater. 25 (2015) 5739– 5747. [11] C.M. Magdalane, K. Kaviyarasu, J.J. Vijaya, C. Jayakumar, M. Maaza, B. Jeyaraj, Photocatalytic degradation effect of malachite green and catalytic hydrogenation by UV– illuminated CeO2/CdO multilayered nanoplatelet arrays: Investigation of antifungal and antimicrobial activities, J. Photochem. Photobiol. B 169 (2017) 110–123. [12] C.M. Magdlane, K. Kaviyarasu, J.J. Vijaya, B. Siddhardha, B. Jeyaraj, Photocatalytic activity of binary metal oxide nanocomposites of CeO2/CdO nanospheres: Investigation of optical and antimicrobial activity, J. Photochem. Photobiol. B 163 (2016) 77–86. [13] K. Kaviyarasu, E. Manikandan, P. Paulraj, S.B. Mohamed, J. Kennedy, One dimensional well-aligned CdO nanocrystal by solvothermal method, J. Alloys Compd. 593 (2014) 67–70. [14] L. Cheng, Q. Xiang, Y. Liao, H. Zhang, CdS-Based photocatalysts, Energy Environ. Sci. 11 (2018) 1362–1391. [15] P. Garg, S. Kumar, I. Choudhuri, A. Mahata, B. Pathak, Hexagonal planar CdS monolayer sheet for visible light photocatalysis, J. Phys. Chem C 120 (2016) 7052–7060. [16] X. Wu, J. Chen, C. Tan, Y. Zhu, Y. Han, H. Zhang, Controlled growth of high-density CdS and CdSe nanorod arrays on selective facets of two-dimensional semiconductor nanoplates, Nat. Chem. 8 (2016) 470–475.
[17] Y.F. Lin, J. Song, Y. Ding, S.Y. Lu, Z.L. Wang, Alternating the output of a CdS nanowire nanogenerator by a whiteǦ lightǦ stimulated optoelectronic effect, Adv. Mater. 20 (2008) 3127–3130. [18] R. Ranjan, M. Kumar, A.S.K. Sinha, Development and characterization of rGO supported CdS–MoS2 photoelectrochemical catalyst for splitting water by visible light, Int. J. Hydrogen Energy 44 (2019) 16176–16189. [19] S. Liu, Z.R. Tang, Y. Sun, J.C. Colmenares, Y.J. Xu, One-dimension-based spatially ordered architectures for solar energy conversion, Chem. Soc. Rev. 44 (2015) 5053–5075. [20] Z.R. Tang, Y. Zhang, Y.J. Xu, One dimensional CdS based materials for artificial photoredox reactions, ACS Appl. Mater. Interfaces 4 (2012) 1512–1520. [21] X.L. Yin, L.L. Li, D.C. Li, Z.J. Li, Y.X. Wang, X.J. Kong, J.S. Zhao, J.H. Jiang, J.C. Qian, D.H.
Pang,
X.X.
Du,
J.M.
Dou,
Noble-metal-free
CdS@MoS2
core-shell
nanoheterostructures for efficient and stabilized visible-light-driven H2 generation, Int. J. Hydrogen Energy 44 (2019) 16657–16666. [22] R. Ranjan, M. Kumar, A.S.K. Sinha, Development and characterization of rGO supported CdS–MoS2 photoelectrochemical catalyst for splitting water by visible light, Int. J. Hydrogen Energy 44 (2019) 16176–16189. [23] K. Kaviyarasu, E. Manikandan, M. Maaza, Synthesis of CdS flower-like hierarchical microspheres as electrode material for electrochemical performance, J. Alloys Compd. 648 (2015) 559–563. [24] J. Choi, D.A. Reddy, N.S. Han, S. Jeong, S. Hong, D.P. Kumar, J.K. Song, T.K. Kim, Modulation of charge carrier pathways in CdS nanospheres by integrating MoS2 and Ni2P for improved migration and separation toward enhanced photocatalytic hydrogen evolution, Catal. Sci. Technol. 7 (2017) 641–649. [25] K. Kombaiah, J.J. Vijaya, L.J. Kennedy, M. Bououdina, K. Kaviyarasu, R.J. Ramalingam, M.A. Munusamy, A. AlArfaj, Effect of Cd2+ concentration on ZnFe2O4 nanoparticles on the structural, optical and magnetic properties. Optik 135 (2017) 190–199. [26] H. Wang, W. Zhu, B. Chong, K. Qin, Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure, Int. J. Hydrogen Energy 39 (2014) 90–99.
[27] Q. Wang, S. Li, J. Qiao, R. Jin, Y. Yu, S. Gao, CdS–CdSe (CdTe) core–shell quantum dots sensitized TiO2 nanotube array solar cells, Sol. Energy. Mater. Sol. Cells 132 (2015) 650– 654. [28] Y. Zhu, R. Wang, W. Zhang, H. Ge, L. Li, CdS and PbS nanoparticles co-sensitized TiO2 nanotube arrays and their enhanced photoelectrochemical property, Appl. Surf. Sci. 315 (2014) 149–153. [29] T. Di, B. Cheng, W. Ho, J. Yu, H. Tang, Hierarchically CdS–Ag2S nanocomposites for efficient photocatalytic H2 production, Appl. Surf. Sci. 470 (2019) 196–204. [30] S. Wang, B. Zhu, M. Liu, L. Zhang, J. Yu, M. Zhou, Direct Z-scheme ZnO/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Appl. Catal. B 243 (2019) 19–26. [31] T. Di, B. Zhu, J. Zhang, B. Cheng, J. Yu, Enhanced photocatalytic H2 production on CdS nanorod using cobalt-phosphate as oxidation cocatalyst, Appl. Surf. Sci. 389 (2016) 775– 782. [32] S. Hong, D.P. Kumar, E.H. Kim, H. Park, M. Gopannagari, D.A. Reddy, T.K. Kim, Earth abundant transition metal-doped few-layered MoS2 nanosheets on CdS nanorods for ultraefficient photocatalytic hydrogen production, J. Mater. Chem A. 5 (2017) 20851–20859. [33] M. Gopannagari, D.P. Kumar, D.A. Reddy, S. Hong, M.I. Song, T.K. Kim, In situ preparation of few-layered WS2 nanosheets and exfoliation into bilayers on CdS nanorods for ultrafast charge carrier migrations toward enhanced photocatalytic hydrogen production. J. Catal. 351 (2017) 153–160. [34] B. Chai, M. Xu, J. Yan, Z. Ren, Remarkably enhanced photocatalytic hydrogen evolution over MoS2 nanosheets loaded on uniform CdS nanospheres, Appl. Surf. Sci, 430 (2018) 523–530. [35] R.K. Chava, J. Yeon, M. Kang, Hydrothermal growth of two dimensional hierarchical MoS2 nanospheres on one dimensional CdS nanorods for high performance and stable visible photocatalytic H2 evolution, Appl. Surf. Sci. 433 (2018) 240–248. [36] J. Li, Y. Peng, X. Qian, J. Lin, Few-layer Co-doped MoS2 nanosheets with rich active sites as an efficient cocatalyst for photocatalytic H2 production over CdS, Appl. Surf. Sci. 452 (2018) 437–442.
[37] Z.W. Zhang, Q.H. Li, X.Q. Qiao, D. Hou, D.S. Li, One-pot hydrothermal synthesis of willow branch-shaped MoS2/CdS heterojunctions for photocatalytic H2 production under visible light irradiation, Chinese J. Catal. 40 (2019) 371–379. [38] J. Wan, R. Wang, L. Liu, J. Fan, E. Liu, X. Gao, F. Fu, A novel approach for high-yield solid few-layer MoS2 nanosheets with effective photocatalytic hydrogen evolution, Int. J. Hydrogen Energy 44 (2019) 16639–16647. [39] X. Jiang, Y. Song, M. Dou, J. Ji, F. Wang, Selective growth of vertically aligned twodimensional MoS2/WS2 nanosheets with decoration of Bi2S3 nanorods by microwaveassisted hydrothermal synthesis: Enhanced photo-and electrochemical performance for hydrogen evolution reaction, Int. J. Hydrogen Energy 43 (2018) 21290–21298. [40] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, MoS2 nanoparticles grown on graphene: An advanced catalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 133 (2011) 7296–7299. [41] D.J. Late, B. Liu, H.R. Matte, V.P. Dravid, C.N.R. Rao, Hysteresis in single-layer MoS2 field effect transistors, ACS Nano 6 (2012) 5635–5641. [42] B. Radisavljevic, A. Radenovic, J. Brivio, I.V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [43] Z. Wang, Z. Jin, G. Wang, B. Ma, Efficient hydrogen production over MOFs (ZIF-67) and g-C3N4 boosted with MoS2 nanoparticles, Int. J. Hydrogen Energy 43 (2018) 13039–13050. [44] O. Khaselev, J.A. Turner, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science 280 (1998) 425–427. [45] L.Q. Jing, Y.G. Xu, Z.G. Chen, M.Q. He, M. Xie, J. Liu, H. Xu, S.Q. Huang, H.M. Li, Different morphologies of SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation, ACS Sustainable. Chem. Eng. 6 (2018) 5132–5141. [46] P.
Sinsermsuksakul, L.
Sun, S.W.
Lee, H.H.
Park, S.B.
Kim, C.
Yang, R.G.
Gordon, Overcoming efficiency limitations of SnS2Ǧ based solar cells, Adv. Energy Mater. 4 (2014) 1400496.
[47] H.W. Chen, Y.T. Chen, H. Zhang, D.W. Zhang, P. Zhou, J. Huang, Suspended SnS2 layers by light assistance for ultrasensitive ammonia detection at room temperature, Adv. Funct. Mater. 28 (2018) 1801035.
[48] W.P. Sun, X.H. Rui, D. Yang, Z.Q. Sun, B. Li, W.Y. Zhang, Y. Zong, S. Madhavi, S. Dou, Q.Y. Yan, Two-dimensional tin disulfide nanosheets for enhanced sodium storage, ACS Nano 9 (2015) 11371–11381. [49] Y. Liu, Y. Yang, X. Wang, Y. Dong, Y. Tang, Z. Yu, Z. Zhao, J. Qiu, Flexible paper like free standing electrodes by anchoring ultrafine SnS2 nanocrystals on graphene nanoribbons for high-performance sodium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 15484– 15491. [50] M.A. Lukowski, A.S. Daniel, C.R. English, F. Meng, A. Forticaux, R.J. Hamers, S. Jin, Highly active hydrogen evolution catalysis from metallic WS2 nanosheets, Energy Environ. Sci. 7 (2014) 2608–2613. [51] D.L. Greenaway, R. Nitsche, Preparation and optical properties of group IV–VI2 chalcogenides having the CdI2 structure, J. Phys. Chem. Solids 26 (1965) 1445–14458. [52] Z. Zhang, C. Shao, X. Li, Y. Sun, M. Zhang, J. Mu, Y. Liu, Hierarchical assembly of ultrathin hexagonal SnS2 nanosheets onto electrospun TiO2 nanofibers: Enhanced photocatalytic activity based on photoinduced interfacial charge transfer, Nanoscale 5 (2013) 606–618. [53] Z. Zhang, J. Huang, M. Zhang, Q. Yuan, B. Dong, Ultrathin hexagonal SnS2 nanosheets coupled with g-C3N4 nanosheets as 2D/2D heterojunction photocatalyts toward high photocatalytic activity, Appl. Catal. B 163 (2015) 298–305. [54] J. Low, J. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [55] Q. Xu, L. Zhang, J. Yu, S. wageh, A. A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: Principles, synthesis, and applications, Mater. Today 21 (2018) 1042–1063. [56] Y. Huo, Y. Yang, K. Dai, J. Zhang, Construction of 2D/2D porous graphitic C3N4/SnS2 composite as a direct Z-scheme system for efficient visible photocatalytic activity, Appl. Surf. Sci. 481 (2019) 1260–1269. [57] Y. Huo, J. Zhang, K. Dai, Q. Li, J. Lv, G. Zhu, C. Liang, All-solid-state artificial Z-scheme porous g-C3N4/Sn2S3-DETA heterostructure photocatalyst with enhanced performance in photocatalytic CO2 reduction, Appl. Catal. B 241 (2019) 528–538. [58] T. Hu, P. Li, J. Zhang, C. Liang, K. Dai, Highly efficient direct Z-scheme WO3/CdSdiethylenetriamine photocatalyst and its enhanced photocatalytic H2 evolution under visible light irradiation, Appl. Surf. Sci. 442 (2018) 20–29.
[59] J. Low, B. Dai, T. Tong, C. Jiang, J. Yu, In situ irradiated XǦ ray photoelectron spectroscopy investigation on a direct ZǦ Scheme TiO2/CdS composite film photocatalyst, Adv. Mater. 31 (2018) 1802981. [60] E. Huang, X. Yao, W. Wang, G. Wu, N. Guan, L. Li, SnS2 nanoplates with specific facets exposed for enhanced visibleǦ lightǦ driven photocatalysis, ChemPhotoChem 1 (2017) 60– 69. [61] K. Zhang, M. Fujitsuka, Y. Du, T. Majima, 2D/2D heterostructured CdS/WS2 with efficient charge separation improving H2 evolution under visible light irradiation, ACS Appl. Mater. Interfaces 10 (2018) 20458–20466. [62] K. Kaviyarasu, P.A. Devarajan, S.S.J. Xavier, S.A. Thomas, S. Selvakumar, One pot synthesis and characterization of cesium doped SnO2 nanocrystals via a hydrothermal process, J. Mater. Sci. Technol. 28 (2012) 15–20. [63] K. Kaviyarasu, E. Manikandan, J. Kennedy, M. Jayachandran, R. Ladchumananandasiivam, U.U. De Gomes, M. Maaza, Synthesis and characterization studies of NiO nanorods for enhancing solar cell efficiency using photon upconversion materials, Ceramics Int. 42 (2016) 8385–8394. [64] J. Yu, J. Jin, B. Cheng, M. Jaroniec, A noble metal-free reduced graphene oxide–CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel, J. Mater. Chem. A. 2 (2014) 3407–3416. [65] Q. Xiang, B. Cheng, J. Yu, GrapheneǦ based photocatalysts for solarǦ fuel generation, Angew. Chem. Int. Ed. 54 (2015) 11350–11366. [66] C.M. Magdalane, K. Kaviyarasu, A. Raja, M.V. Arularasu, G.T. Mola, A.B. Isaev, N.A. AlDhabi, M.V. Arasu, B. Jeyaraj, J. Kennedy, M. Maaza, Photocatalytic decomposition Photocatalytic decomposition effect of erbium doped cerium oxide nanostructures driven by visible light irradiation: Investigation of cytotoxicity, antibacterial growth inhibition using catalyst, J. Photochem. Photobiol. B 185 (2018) 275–282. [67] A.A. Ezhilarasi, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, R.J. Ramalingam, H.A. AlLohedan, Green synthesis of NiO nanoparticles using Aegle marmelos leaf extract for the evaluation of in-vitro cytotoxicity, antibacterial and photocatalytic properties, J. Photochem. Photobiol. B 180 (2018) 39–50.
[68] S.K. Jesudoss, J.J. Vijaya, P.I. Rajan, K. Kaviyarasu, M. Sivachidambaram, L.J. Kennedy, H.A. Al-Lohedan, R. Jothiramalingam, M.A. Munusamy, High performance multifunctional green Co3O4 spinel nanoparticles: Photodegradation of textile dye effluents, catalytic hydrogenation of nitro-aromatics and antibacterial potential, Photochem. Photobiol. Sci. 16 (2017) 766–778. [69] W. Zhang, Y. Wang, Z. Wang, Z. Zhong, R. Xu, Highly efficient and noble metal-free NiS/CdS photocatalysts for H2 evolution from lactic acid sacrificial solution under visible light, Chem. Commun. 46 (2010) 7631–7633.
SCHEMES AND FIGURES
Scheme 1. Preparations of CdS nanorods, SnS2 nanosheets and CdS/SnS2 nanocomposites.
100
200
300
400
500
Intensity (a. u.)
Cd 3p3 Cd 3p1
Sn 3d O 1s
C 1s
S 2s
Sn 4d Sn 4p Sn 4s S 2p
Intensity (a. u.) 0
S2p3/2
(b) S 2p Cd 3d
(a)
600
700
800
S2p1/2
156 157 158 159 160 161 162 163 164 165 166
Binding energy (eV)
(d) Cd 3d
Sn 3d3/2
Intensity (a. u.)
Sn 3d5/2
Intensity (a. u.)
(c) Sn 3d
Binding energy (eV)
Cd3d5/2 Cd3d3/2
480 482 484 486 488 490 492 494 496 498 500
400 402 404 406 408 410 412 414 416 418
Binding energy (eV)
Binding energy (eV)
Fig. 1. X-ray photoelectron spectra of CdS/SnS2 nanocomposites: (a) Survey spectrum and (b)–(d) expanded spectra of S 2p, Sn 3d and Cd 3d regions, respectively.
Fig. 2. (a) TEM image, (b, c) HRTEM images and (d)–(i) elemental analyses of Cd, Sn and S elements, respectively, of CdS/SnS2 nanocomposites.
Fig. 3. (a) UV–visible diffuse reflectance spectra, (b) PL spectra, (c) PEC analyses and (d) electrochemical impedance spectra of CdS nanorods and CdS/SnS2 nanocomposites.
(b)
35 (c) -1
Amount of H2 (mmol. h )
3rd Reproducibility
-1
-1
Amount of H2 (mmol. g . h )
(a) 20 15 10 5
2nd
1st
0 2
3
5
6
7
10
0
20 15 10 5
10
Lactic acid addition
20
0.112
0 0
1
2
3
4
5
Lactic acid (mL)
-1
15 10 5
3
4
5
6
Catalyst dosage (mg)
Amount of H2 (mmol. g )
-1
10
2
-1
-1
15
1
20
500
(e) Amount of H2 (mmol. g . h )
20 (d) -1
15
Amount of H2 (mmol. g . h )
2
-1
5
-1
Amount of SnS loading on CdS (wt%)
Amount of H2 (mmol. g . h )
25
0 0
5
30
(f)
400 Ar purged
300 200 100 0
0 1st
2nd
3rd
4th
Number of cycles
5th
0
5
10
15
20
25
30
35
Irradiation time (h)
Fig. 4. Photocatalytic assessments of optimized CdS/SnS2 nanocomposites: (a) effect of SnS2 loading wt. onto CdS nanorods, (b) reproducibility tests, (c) effects of the catalyst dosage and (d) scavenger effect on photocatalytic performance of the CdS/SnS2 nanocomposites, (e) recyclability of CdS/SnS2 nanocomposite and (f) long-term stability of the CdS/SnS2 nanocomposite under continuous photo-irradiation.
Scheme 2. Proposed photocatalytic H2 production mechanism of CdS/SnS2 nanocomposites with band-gap structures under simulated solar irradiation.
!"#$%&%'()*+),+(*'"&"-'-+ ! !*!"#$!%&'#()*!+$,-%)$!'#%'!'#$.!#%/$!0(!10(20!,(34$'506!750%0,5%-!50'$)$*'*!()!4$)*(0%-!)$-%'5(0*#54܈ !'#%'!,(&-+!#%/$!%44$%)$+!'(!507-&$0,$!'#$!2()1!)$4()'$+!50!'#5*!4%4$)8 ! !!"#$!%&'#()*!+$,-%)$!'#$!7(--(2506!750%0,5%-!50'$)$*'*94$)*(0%-!)$-%'5(0*#54*!2#5,#!3%.!:$!,(0*5+$)$+ !!;*'*%*!4('$0'5%-!,(34$'506!50'$)$ ! !
! ! ! !
!"#$%&'(%)#&*+"#*%)',#-#./.)#' !0' 1"##-' 2-)3-44-!" #$%&'()*+,-.+)-$%/" 0$12+," +%+,34-4/" 5%6'4)-7+)-$%/" 81-)-%7" 9" :1-7-%+," ;1+<)" 50'1&-6..)'7"/-&!"#$%&'()*+,-.+)-$%/"0$12+,"+%+,34-4/"5%6'4)-7+)-$%/"81-)-%7"9"='6-'>"+%?" @?-)-%7" 8-9$":"9-)-';%4-))-3-&*!"0$12+,"+%+,34-4/"A+,-?+)-$%" 50'!/-&-)-#$-'2.99'?%)3!"A+,-?+)-$%/"A-4*+,-.+)-$%" ="@*)'7*/!"A+,-?+)-$%/"A-4*+,-.+)-$%" A-.' 7<"' 7*/!" #$%&'()*+,-.+)-$%/" B*('16-4-$%/" 81-)-%7" 9" ='6-'>" +%?" @?-)-%7/" 0*%?-%7" C&D*-4-)-$%" "
Graphical Abstract
·
Optimal hydrogen evolution rate of 20.2 mmol h−1 g−1 facilitated by enhanced absorption and prolonged life time of charge carriers by SnS2 as co-catalyst on CdS.
!"#"$%&'()*+',*+'-#( ( !! !"#$%&'%()*+%',$-($.'/"0&'-(%10$(2)&3)(&4(-&-$(5'/+4()&,)$%(0)1-(0)$(6+!7( !! 893$//$-0(4$#1%10&'-('5(#)'0'(,$-$%10$+(3)1%,$(31%%&$%4('-(6+!:!-!;()*<%&+(2&0)(!-!;( &-4$%0&'-7( !! 893$//$-0(#)'0'(401<&/&0*('5(0)$(3101/*40(5'%(/'-,(0$%=(#%130&31/(1##/&310&'-4(%$=1&-$+( $55$30&.$(5'%(>?()("-+$%('#0&=&@$+(3'-+&0&'-47( !! A-&5'%=(+$#'4&0&'-('5("/0%10)&-(!-!;(4)$$04('-0'(6+!(<*(4&=#/$(=$0)'+47(
S0169-4332(19)33619-0 https://doi.org/10.1016/j.apsusc.2019.144803 APSUSC 144803
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
16 August 2019 18 November 2019 19 November 2019
Please cite this article as: A. Putta Rangappa, D. Praveen Kumar, M. Gopannagari, D. Amaranatha Reddy, Y. Hong, Y. Kim, T. Kyu Kim, Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.144803
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Highly efficient hydrogen generation in water using 1D CdS nanorods integrated with 2D SnS2 nanosheets under solar light irradiation A. Putta Rangappa1, D. Praveen Kumar1, Madhusudana Gopannagari, D. Amaranatha Reddy, Yul Hong, Yujin Kim, and Tae Kyu Kim* Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea
Abstract The development of low-cost and noble-metal-free catalysts for the photoconversion of water into hydrogen (H2) is of great interest. Here, 2D tin(IV) sulfide (SnS2) ultrathin nanosheets as coǦ catalysts are coupled with 1D cadmium sulfide (CdS) nanorods for the photosplitting of water into H2. The design of the catalyst can facilitate the passivation of the physiochemical properties of CdS and enhance H2 evolution activity. The prepared CdS/SnS2 composite catalyst increases the H2 generation activity and exhibits excellent and continuous long-term photostability. The H2 evolution rate of the optimized CdS/SnS2 composite is approximately 9-fold that of pristine CdS nanorods. The characterization results of the CdS/SnS2 composite reveal that the loading of SnS2 can enhance the synergistic effects of the photocatalyst due to the effective separation, large number of exposed catalytic sites, and highly dispersed nature of the layered SnS 2. Several characterization outcomes of CdS/SnS2 are examined in detail (e.g., structural and surface elemental results of transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy). Further, the optoelectrical properties and charge-carrier excitations
are investigated via ultraviolet diffuse reflectance spectroscopy, photoluminescence spectroscopy, and photoelectrochemical analysis. The proposed CdS/SnS2 composite is a promising low-cost, noble-metal-free, and high-efficiency catalyst for the photocatalytic water-reduction reaction. Keywords: SnS2 ultrathin nanosheets, 1D/2D system, hydrogen production, water splitting, charge-transfer. *Corresponding author. E-mail address: [email protected] (T.K. Kim) 1
The author contributed equally to this work.
1. Introduction The energy demand is predicted to double over the next 30 years, and at present, the main sources of energy production are fossil fuels. However, continued dependence on fossil fuels to satisfy energy requirements will lead to significant global warming and environmental pollution. Additionally, fossil fuel resources are rapidly being depleted; therefore, it necessary to satisfy the growing energy requirements in a sustainable way, without harming the environment. Solar energy is a continuous and abundant source but is presently not storable [1–4]. Hence, there is an urgent need to develop an approach for storing solar energy. The photosplitting of water into hydrogen (H2) and oxygen (O2) utilizing solar energy is considered one of the effective ways to preserve solar light. The generation of H2 from solar-driven water reduction is an efficient, clean, and sustainable approach. Photocatalytic reactions mediated by semiconductors can harvest solar energy and convert it into chemical energy. These processes can be utilized in various important environmental chemistry applications [5–7].
Several semiconductor materials have been explored for photocatalytic water splitting reactions. Among them, oxide-based materials with wide bandgaps have limitations in utilizing a broad spectrum; they only absorb light in the ultraviolet (UV) region of the solar spectrum [8]. Nitrides have poor aqueous stability [9]. The synthesis of phosphides involves a high temperature and the flammable element phosphorus, and phosphides have limited applications. Sulfide (S)based semiconductors are interesting owing to their distinct optoelectronic properties [10]. It is highly desirable to select materials with suitable bandgaps and band potentials for achieving high photocatalytic efficiencies for H2 production via the effective absorption of visible light. In this regard, cadmium sulfide (CdS) is one of the well-known S-based visible-light active semiconductor photocatalysts, and it is easy to prepare in nanostructures [11–14]. Various nanostructures of CdS have been used in diverse applications, such as sensors, photovoltaic cells, solar cells, photoluminescent strategies, and electro-optic modulators [15–17]. Different CdSbased nanostructures have been developed with valence- and conduction-band edges tuned for applications in diverse fields. However, the utilization of 2D films and 0D nanoparticles is limited in photocatalysis owing to the lack of continuous conducting pathways. It is believed that 1D nanostructures have better charge-transport properties [18–20]. CdS nanorods have great potential as catalysts, and they have attracted considerable attention for photocatalytic H2 evolution because of their excellent water-reduction potential and high charge-separation efficiency on the surface [21,22]. However, the CdS semiconductor is not highly active for wavelengths above 520 nm, limiting the utilization of solar light. Additionally, the fast charge-carrier recombination and photocorrosion reduce the efficiency of bare CdS for photocatalytic applications [23–25]. Thus, researchers have attempted to co-sensitize CdS with different suitable co-catalysts (e.g., CdSe,
CdTe, PbS, Ag2S, ZnO, Co3(PO4)2) to attain high photocatalytic performance [26–31]. Recently, 2D co-catalyzed CdS has been widely investigated owing to its peculiar properties [32–37]. Among the materials, 2D materials are the most popular, and they exhibit fascinating properties. Promising alternative co-catalysts include 2D transition-metal dichalcogenides (TMDs), which exhibit distinctive properties, such as direct bandgaps, large atomic thickness, strong spin–orbit coupling, and advantageous electronic and mechanical characteristics, making them interesting for basic energy conversion research. Among the various semiconducting TMDs, such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), and tin(IV) sulfide (SnS2), MoS2 and WS2 are important TMD semiconductor materials owing to their attractive electrical properties, and thus, they have been widely investigated. There is a need to develop highly efficient, stable photocatalysts. However, the poor charge transfer, low stability, and hydrophobic characteristics of MoS2 [38–44] and the low density and reactivity of active sites and limited corrosion stability of WS2 are major disadvantages for photocatalytic reactions [45]. The TMD SnS 2 (IV–VI metal sulfide semiconductor) with bandgap 2.0–2.5 eV [46] is widely applicable in diverse fields (e.g., photocatalysis, solar cells, sensors and batteries) due to its excellent properties, such as its high optical absorption coefficient, tunable bandgap, and natural abundance [47–50]. SnS2 can be an excellent alternative for photocatalytic applications owing to its good stability in both acidic and alkaline solutions, high thermal stability, and nontoxicity [51,52]. The exceptional range of visiblelight absorption of SnS2 allows the construction of heterojunctions with SnS2 for photocatalytic water splitting into H2. Different types of heterostructures have been reported with SnS 2 as a cocatalyst, confirming that the formation of heterostructures with SnS2 for effective charge separation is beneficial for photocatalytic applications [53, 54]. Photocatalytic systems with the direct Z-scheme process for enhanced photocatalytic activity have been reported [55–59]. The
CdS/SnS2 nanohybrid, which facilitates electron/hole transport, suppresses the charge recombination of CdS and promotes photocatalytic water splitting H2 evolution. In this study, we developed a low-cost, highly efficient, and stable semiconductor CdS/SnS2 nanohybrid via facile synthesis techniques and employed it for photocatalytic water splitting H2 evolution. This nanohybrid structure exhibited a robust H2 production rate of 20.2 (mmol h−1 g−1) while under photoirradiation for 3 h. This activity was significantly better than that of bare SnS 2 and 9-fold higher than that of pristine CdS. More importantly, the CdS/SnS2 nanohybrid catalyst exhibited long-term stability over 30 h with continuous photoirradiation, indicating its potential for long-term practical applications. The plausible mechanism of CdS/SnS2 for photocatalytic H2 generation was elucidated along with the effective charge separation based on the band alignment between CdS and SnS2. This study provides insights regarding the development of inexpensive and noble-metal-free semiconductor photocatalysts for the H2 evolution reaction system under solar light.
2. Experimental 2.1. Raw Materials Thioacetamide-TAA
(C2H5SN),
tin
(IV)
chloride
pentahydrate
(SnCl4∙5H2O),
polyvinylpyrrolidone (PVP; molecular weight: 30000 g·mol−1), cadmium acetate dihydrate (Cd(AC)2∙2H2O), ethylene glycol (EG, (C2H6O2)), ethylenediamine anhydrous (EDA, C2H8N2), thiourea (CH4N2S), ethanol, N,N-dimethylformamide (DMF), lactic acid (LA), and deionized (DI) water were used. 2.2. Preparation of CdS Nanorods
In the preparation process, 20.25 mmol of Cd(AC)2∙2H2O and 60.75 mmol of CH4N2S were suspended in 60 mL of EDA and ultrasonicated for 1 h to ensure a consistent dispersion. This dispersion was transferred to a clean autoclave (Teflon vessel) and heated to 160°C for 48 h. Then, the reaction mass was cooled to 25–27°C. The resulting yellow pasty mass was collected and cleaned thoroughly with DI water and then ethanol. Finally, the obtained material was dried in a hot air oven at 80°C for 10–12 h. 2.3. Preparation of SnS2 Ultrathin Nanosheets In the synthesis process, SnCl4∙5H2O and C2H5SN with a mass ratio of 1:2, as well as 2 g of PVP, were suspended in 50 mL of EG. The suspension was vigorously stirred for 30 min to obtain a homogeneous dispersion. Then, the mixture was transferred to an autoclave and treated hydrothermally at 160°C for 12 h. After the completion of the reactions, the obtained product was cleaned thoroughly with DI water and then ethanol and dried in a hot air oven at 80°C for 10–12 h. 2.4. Preparation of CdS/SnS2 Nanohybrid Structure The CdS/SnS2 nanohybrid structure was prepared via the solvent exfoliation method in DMF. In this process, the required amount of bulk SnS2 was first dispersed in the solvent (10 mL of DMF) and sonicated for 2 h to achieve exfoliation from the bulk SnS2. CdS nanorods were added, and the sonication was continued for 1 h. The well-dispersed products were stirred for 12 h at room temperature to deposit SnS2 ultrathin nanosheets onto the CdS nanorods. The obtained products were cleaned with DI water followed by ethanol several times until the solvent was removed completely. Then, the products were dried at 80Ԩ for 10–12 h. The overall synthesis procedures for SnS2, CdS, and CdS/SnS2 nanohybrids are illustrated in Scheme 1.
2.5. Characterization Details of Synthesized Products The photocatalyst powder was characterized as follows. X-ray diffraction (XRD) patterns were collected using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed using Al Kα radiation (1486.6 eV) with an energy of 15 kV/150 W. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV. Diffuse reflectance spectra were recorded using a UV-1800 SHIMADZU, and photoluminescence (PL) spectra were measured using a Hitachi F-7000 florescence spectrometer. Photoelectrochemical (PEC) experiments were performed using a CHI 617B unit with a threeelectrode cell. Platinum wire was used as a counter electrode, and silver/silver chloride (Ag/AgCl) was used as a reference electrode. A solar simulator with an AM 1.5G filter and a 150-W xenon (Xe) lamp (ABET Technologies) was used as the light source. The silicon (Si) reference cell was used to adjust the light intensity to 1 sun (100 W/m2). A 0.5 M solution of Na2SO4 was used as the electrolyte. The working electrode was prepared by cleaning an In-doped SnO2 (ITO)-coated glass with ethanol. The photocatalyst (10 mg) was dispersed in ethanol (450 μL) and Nafion (50 μL) via soft ultrasonication, and the uniform suspension (30 μL) was added dropwise directly onto the cleaned ITO glass substrate using a micro-syringe. The substrate and suspension were placed in an oven at 100°C for 3 h to accelerate the drying. The solar simulator was used for the measurement of photoresponses through on–off cycling. Mott–Schottky (MS) measurements were performed under dark conditions at a frequency of 1 kHz using an impedance spectra analyzer with a fixed potentiostat. The other conditions were identical to those used for the PEC experiments. The
obtained potentials vs. Ag/AgCl were transformed to the normal hydrogen electrode (NHE) scale using the equation ENHE = EAg/AgCl + 0.197 (V) at a pH of 7. 2.6. Photocatalytic Assessments Photocatalytic experiments to generate H2 were performed in a quartz reactor (150 mL) containing a solution of 15 mL of water, 20 vol. % hole scavenger LA, and 1 mg of synthesized material. The reactor was closed tightly with an airtight rubber septum after all the materials were added. The solar simulator with an AM 1.5G filter and a 150-W Xe lamp (ABET Technologies) was used as the light source. The Si reference cell was used to adjust the light intensity to 1 sun (100 W/m 2). Before commencing irradiation, the system was evacuated for 30 min and degasified with Ar for 30 min to nullify the presence of air in the system. The amount of H2 produced was evaluated by an offline gas chromatograph (Young Lin Autochro-3000) equipped with a thermal conductivity detector.
3. Results and Discussion 3.1. Structural Analyses The crystal structures and phases of the as-synthesized CdS, SnS2, and CdS/SnS2 were analyzed via powder XRD. Fig. S1 in Supplementary Data (SD) shows the diffraction peaks of CdS, SnS 2, and the CdS/SnS2 nanohybrid structure. The as-prepared CdS exhibited a hexagonal lattice structure and strongly matched JCPDS Card #89-2944. No peaks other than that corresponding to CdS were observed. The diffraction peaks of SnS2 confirmed the existence of a hexagonal lattice structure. The peaks at 28.22°, 32.11°, 52.45°, 59.55°, and 70.35° corresponded to the planes of (100), (101), (111), (112), and (113), respectively, closely matching the structure of SnS2 (JCPDS
Card # 40-1467). The high-intensity peak at 52.45° was attributed to the (101) plane of the hexagonal SnS2 nanosheets, and the diffraction peaks corresponding to the (100) and (101) planes had high intensities compared with the standard values. The results indicated the preferential orientation of the (101) plane. This distinct orientation of planes resulted from the capping of PVP. SnS2 nanostructures synthesized with appropriately exposed facets can enhance visible-lightsensitive photocatalysts [60]. The diffraction peaks of SnS2 were undetectable for the composite, indicating the relatively low content of SnS2 products in the CdS/SnS2 nanohybrid structure. Moreover, we noticed a slight increase in the diffraction intensity of CdS/SnS2 compared with bare CdS, probably due to the higher atomic scattering factor. 3.2 Morphology and Surface Elemental Analysis The chemical states and elemental composition of the optimized CdS/SnS2 nanocomposite were examined via XPS, as shown in Fig. 1. The XPS spectra clearly indicate the presence of Sn, Cd, and S in the composite. Additionally, the elements and their chemical states were confirmed by the high-resolution Cd 3d, Sn 3d, and S 2p X-ray photoelectron spectra. Fig. 1(a) shows the complete survey spectrum, and Figs. 1(b)–(d) show the high-resolution S 2p, Sn 3d, and Cd 3d XPS spectra of the CdS/SnS2 nanohybrid structure, respectively. The peaks of the Cd 3d spectrum were observed at binding energies 405.1 and 411.9 eV, corresponding to Cd 3d5/2 and Cd 3d3/2, respectively. The peaks of Sn 3d and S 2p appeared at 486.7 eV (Sn 3d5/2) and 495.0 eV (Sn 3d3/2) and at 161.2 eV (2 p3/2) and 162.4 eV (2 p1/2), respectively. When CdS was coupled with SnS2, the binding energy of Cd 3d was blue-shifted (Fig. S4), which is an indication of the strong interactions between CdS and SnS2. Similarly, Zhang et al. also reported the Cd 3d XPS spectrum shift for the CdS/WS2 nanocomposite [61].
The TEM image of pure SnS2 clearly confirms the nanosheets’ morphology and image, as displayed in Fig. S3. The morphologies of the samples were analyzed via TEM, as shown in Fig. 2. The morphology of the composite clearly indicated that the ultrathin SnS2 sheets were uniformly deposited onto the CdS nanorods. The exfoliated SnS2 sheets covered most of the CdS nanorods, increasing the amount of exposed active sites for photocatalytic applications. The HRTEM images of the optimized CdS/SnS2 (Fig. 2(b)) clearly show the presence of the ultrathin nanosheets of SnS2 on the CdS nanorods. Additionally, the elemental composition of the optimized hybrid catalyst was analyzed via energy-dispersive X-ray spectroscopy. As shown in Fig. 2(c), the CdS/SnS2 composite exhibited homogeneous distributions of Cd and S, as well as a relatively weak Sn signal, which mostly appeared on the surfaces of the CdS nanorods. In addition, the HRTEM image of CdS/SnS2 composite clearly confirms the presence of a few layered SnS2 nanosheets on the CdS nanorods by the well-matched lattice fringes of individual materials. However, we verified the presence of the few layered SnS2 nanosheets on the CdS nanorods by EDS mapping, and the results are displayed in Figs. 2(d)–(f). All these findings indicate the successful synthesis of the CdS/SnS2 nanocomposite, where the CdS nanorods were covered by the ultrathin SnS2 nanosheets. 3.3. Optical Studies The optical absorption of CdS and the composite products was investigated via UV diffuse reflectance spectroscopy (UV-DRS). As shown in Fig. 3(a), the CdS and hybrid CdS/SnS2 samples exhibited the classical semiconductor light-absorption feature. In the optical range of >520 nm, the CdS samples with SnS2 exhibited significantly more light absorption compared with CdS. The bandgaps of CdS and composite were calculated to be 2.3 eV. However, compositing CdS with SnS2 may not alter the inherent band structure of the CdS semiconductor; instead, the strong
interaction between the materials leads to increased photoabsorption. The increased visible-light absorption significantly contributed to the harvesting of photons, which typically enhances the H2 generation. UV-DRS was further analyzed for pristine SnS2 (results displayed in Fig. S5), and its bandgap was then calculated (2.08 eV). The charge-carrier transfer and recombination capability of the CdS and composite materials were examined via PL spectroscopy, as shown in Fig. 3(b). The spectra were obtained with an incident excitation wavelength of 380 nm. A strong emission peak around 550 nm was observed for both CdS and CdS/SnS2, owing to the band edge emission property of the CdS nanorods. The reduced peak intensity of the composite indicates the effective transfer and separation of photoinduced charge carriers [62–64]. Therefore, the sensitization of CdS with SnS2 accelerated the excited electron transfer onto the surface, with excellent separation of charge carriers on the composite, and enhanced the photocatalytic H2 production. A PEC analysis was performed to investigate the charge separation on CdS and the composite. The spectra shown in Fig. 3(c) were obtained using an electrochemical potentiostat with working electrodes coated with CdS and CdS/SnS2 nanohybrids on an ITO glass substrate. As shown in Fig. 3(c), the photocurrent intensity of each spectrum increased when the simulated light was turned on, owing to the electrons induced by the light. Additionally, as expected, the CdS/SnS2 nanohybrid exhibited a higher intensity than bare CdS. This enhanced photocurrent indicates the good separation of migrated charge carriers on CdS/SnS2. All the electrons moved to the surface of the catalyst and participated in the reduction of H+ to H2, yielding a large amount of H2. Electrochemical impedance analysis of the CdS nanorods and CdS/SnS2 nanohybrid was performed, as shown in Fig. 3(d). The small semicircular region was smaller for the CdS/SnS 2
nanohybrid than for the bare CdS nanorods, indicating the faster interfacial charge transfer of the CdS/SnS2 nanohybrid. Together, the experimental results suggest that the CdS/SnS2 nanohybrids had enhanced catalytic activity compared with the bare CdS nanostructures under solar light. The observed features strongly indicate that the SnS2 nanosheets greatly influenced the H2 evolution rate of the CdS nanorods under solar light irradiation. 3.4. MS Analysis To elucidate the mechanism underlying the photocatalytic H2 production, an MS analysis was performed to evaluate the exact potentials (conduction band (CB)) of the ultrasonicated CdS nanorods, SnS2 nanosheets, and CdS/SnS2 nanohybrid structures, as shown in Figs. S2(a)–(e) of SD. The band potentials of CdS/SnS2, CdS, and SnS2 were estimated to be -0.958, -1.195, and 0.81 V (vs. EAg/AgCl), respectively. The obtained potentials vs. Ag/AgCl were transformed to the NHE scale using the equation ENHE = EAg/AgCl + 0.197 (V) at pH = 7 [65]. The CB potentials (vs. ENHE) were calculated to be -0.761, -0.998, and -0.613 V for CdS/SnS2, CdS, and SnS2, respectively. The CB potential of CdS/SnS2 was between those of CdS and SnS2. It was more negative than the H+ reduction potential (–0.410 V vs. ENHE at a pH of 7) and more positive than the CB potential of bare CdS nanorods, indicating the efficient utilization of photogenerated charge carriers, which enhanced the photocatalytic H2 production. 3.5. Photocatalytic Activity The photocatalytic activity of CdS/SnS2 for H2 generation was evaluated in the presence of LA under solar light. Fig. 4(a) shows the weight percentage (wt.%) effect of SnS 2 on CdS regarding the photocatalytic H2 performance. The SnS2 loading amount of 5 wt.% yielded the highest H2 production rate (20.2 mmol h−1 g−1), which was enhanced by a factor of 9.2 compared with that of
pristine CdS. Lower activity was observed for larger amounts of SnS2 loading (>5 wt.%), owing to the minimized active sites of CdS with fully covered SnS2 nanosheets. The obtained results were compared with previous reports by providing a comparison table for H2 evolution rates of different CdS-based composites, and they are displayed in Table S1. The reproducibility assessment shown in Fig. 4(b) indicates the high consistency of the H2 evolution rate of the CdS/SnS2 hybrid catalyst. The photocatalytic activity catalyzed by 5 wt.% CdS/SnS2 catalyst in H2 generation was investigated, as shown in Fig. 4(c). Different amounts of catalyst ranging from 1 to 6 mg were used to perform the experiments under identical conditions. As the amount of photocatalyst (5 wt.% CdS/SnS2) increased from 1 to 4 mg in the photocatalytic system (20% LA in 15 mL of reaction solution), the H2 production rate increased to 34.7 mmol h−1, as shown in Fig. 4(c). Saturation of the H2 evolution was observed for the system in which the catalyst amount was >4 mg, because an excessive amount of catalyst in the system shielded the dispersed photocatalyst particles. Additionally, the effect of the scavenger (LA) volume on the H2 generation rate was evaluated for the optimized CdS/SnS2 photocatalyst. Different experiments were conducted in which the LA volume was varied from 0 to 5 mL in 15 mL of the reaction solution. The optimum H 2 evolution rate of 20.2 mmol h−1 g−1 was achieved with the photocatalytic system comprising 3 mL of LA. A further increase in the LA concentration reduced the H2 generation rate, possibly because the presence of an excessive amount of scavenger (i.e., >3 mL) in the system led to a high rate of intermediate formation. Recycling experiments (Fig. 4(e)) provided the H2 generation rate on CdS/SnS2 for five repeated cycles. Each cycle was conducted under identical conditions, and the rate of H2 production significantly decreased after the third cycle, possibly owing to the reduced scavenger concentration in the photocatalytic system [66–68].
However, when an excessive amount (3 mL) of LA was added to the system, the H 2 generation rate approached that observed in the first cycle. This suggests that the consumption of LA through oxidation to pyruvic acid limited the H2 evolution rate in the photocatalytic reaction [69]. The stability of the optimized CdS/SnS2 nanocomposite catalyst was investigated by continuously measuring the H2 production rate under continuous solar light irradiation. As shown in Fig. 4(f), the CdS/SnS2 photocatalyst exhibited high stability for 30 h, indicating its potential for long-term practical applications. A small decline in the H2 production rate was observed after 30 h, possibly owing to the large number of products deposited on the catalyst, as well as the deficiency of the scavenger in the system, which was converted into pyruvic acid via oxidation. Additionally, the photoreactor may have been filled with a large amount of H2 gas, which was generated by a continuous reaction. With repeated evacuation and Ar purging of the system after 30 h, the H2 generation rate for the next 3 h of the photoreaction remained nearly unchanged and was identical to that observed for the first 3 h of the photoreaction. Additionally, we performed photocatalytic studies for simple ultrasonicated CdS, SnS2, and their simple mixer, and they showed a much lower rate of H2 production than the CdS/SnS2 composite due to the high recombination in pristine materials and very weak interactions in the simple mixing of pristine materials. 3.6. Photocatalytic Mechanism According to the analysis results, a plausible mechanism for water reduction into H2 catalyzed by the CdS/SnS2 photocatalyst was proposed (Scheme 2). The electron–hole pairs were generated on CdS/SnS2 under the illumination of solar light. The UV-DRS results revealed the excellent visiblelight absorption capability of the CdS/SnS2 photocatalyst. According to the CB potential levels of
CdS and SnS2, the photogenerated charge carriers (electrons) of CdS were transferred to the CB of SnS2. The lifetime of the electrons was prolonged while the electrons were transferred from CdS to SnS2 through the interfacial contact between CdS and SnS2. The excited electrons rapidly migrated onto the CB of SnS2, efficiently reducing H+ to produce H2. On the other side, the photogenerated holes in the valence band (VB) of SnS2 moved to the VB of CdS, and the scavenger accepted the holes, transforming into pyruvic acid. Thus, the complete cycle of H 2 generation under solar light was accomplished.
4. Conclusion Layered ultrathin SnS2 sheets are attractive noble-metal-free, highly efficient, and stable cocatalysts for photocatalytic H2 generation from water. A high H2 evolution rate was achieved using a CdS/SnS2 photocatalyst, which was 9 times higher than that of bare CdS. Combining highly visible-light active SnS2 nanosheets as co-catalysts with CdS led to a large number of active sites for catalytic applications and enhanced optical properties compared with bare CdS. The CdS/SnS2 photocatalyst exhibited stability for >30 h, with no degradation in the rate of H2 evolution. The development of low-cost and stable photocatalytic systems has attracted considerable attention for the advancement of highly efficient photocatalysts for long-term practical applications. Hence, the proposed system provides significant motivation for further studies on energy and environmental applications. This approach might provide valuable guidance for the development of metal dichalcogenide (MX2) 2D materials.
Supplementary Data
XRD patterns of CdS, SnS2, and CdS/SnS2 nanohybrids; Mott-Schottky measurements of CdS, SnS2, and CdS/SnS2 nanohybrids; TEM image of ultrathin SnS2 nanosheets; High resolution XPS spectra of Cd 3d of CdS and CdS/SnS2 nanohybrids; UV–visible diffuse reflectance spectrum of SnS2, Comparison table of H2 evolution rate of different CdS and SnS2 based nanocomposites.
Acknowledgements Funding: This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (grant numbers: 2016R1E1A1A01941978 and 2016K1A4A4A01922028).
References [1]
N.S. Lewis, D.G. Nocera, Powering the planet: Chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. 103 (2016) 15729–15735.
[2]
W. Wei, H. Wang, Y.H. Hu, Unusual particle-size-induced promoter-to-poison transition of ZrN in counter electrodes for dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 14350– 14357.
[3]
H. Wang, W. Wei, Y. H. Hu, Efficient ZnO-based counter electrodes for dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 6622–66288.
[4]
H. Wang, Y.H. Hu, Graphene as a counter electrode material for dye-sensitized solar cells, Energy. Environ. Sci. 5 (2012) 8182–8188.
[5]
C.M. Magdalane, K. Kaviyarasu, N. Matinise, N. Mayedwa, N. Mongwaketsi, D. Letsholathebe, G.T. Mola, N.A. Dhabi, M.V. Arasu, M. Henini, J. Kennedy, M. Maaza, B. Jeyaraj, Evaluation on La2O3 garlanded ceria heterostructured binary metal oxide nanoplates for UV/visible light induced removal of organic dye from urban wastewater, S. African J. Chem. Eng. 26 (2018) 49–60.
[6]
M. Sivachidambaram, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, H.A. Al-Lohedan, R.J. Ramalingam, A novel synthesis protocol for Co3O4 nanocatalysts and their catalytic applications, RSC. Adv. 7 (2017) 38861–38870
[7]
Y.S. Reddy, C.M. Magdalane, K. Kaviyarasu, G.T. Mola, J. Kennedy, M. Maaza, Equilibrium and kinetic studies of the adsorption of acid blue 9 and Safranin O from aqueous solutions by MgO decked FLG coated Fuller's earth, J. Phys. Chem. Solids 123 (2018) 43– 51.
[8]
K. Maeda, K. Domen, New non-oxide photocatalysts designed for overall water splitting under visible light, J. Phys. Chem C 111 (2017) 7851–7861.
[9]
Y. Wu, P. Lazic, G. Hautier, K. Persson, G. Ceder, First principles high throughput screening of oxynitrides for water-splitting photocatalysts, Energy Environ. Sci. 6 (2013) 157–168.
[10] D.V. Shinde, S.A. Patil, K. Cho, D.Y. Ahn, N. K. Shrestha, R. S. Mane, S. H. Han, Revisiting metal sulfide semiconductors: A solutionǦ based general protocol for thin film formation, hall effect measurement, and application prospects, Adv. Funct. Mater. 25 (2015) 5739– 5747. [11] C.M. Magdalane, K. Kaviyarasu, J.J. Vijaya, C. Jayakumar, M. Maaza, B. Jeyaraj, Photocatalytic degradation effect of malachite green and catalytic hydrogenation by UV– illuminated CeO2/CdO multilayered nanoplatelet arrays: Investigation of antifungal and antimicrobial activities, J. Photochem. Photobiol. B 169 (2017) 110–123. [12] C.M. Magdlane, K. Kaviyarasu, J.J. Vijaya, B. Siddhardha, B. Jeyaraj, Photocatalytic activity of binary metal oxide nanocomposites of CeO2/CdO nanospheres: Investigation of optical and antimicrobial activity, J. Photochem. Photobiol. B 163 (2016) 77–86. [13] K. Kaviyarasu, E. Manikandan, P. Paulraj, S.B. Mohamed, J. Kennedy, One dimensional well-aligned CdO nanocrystal by solvothermal method, J. Alloys Compd. 593 (2014) 67–70. [14] L. Cheng, Q. Xiang, Y. Liao, H. Zhang, CdS-Based photocatalysts, Energy Environ. Sci. 11 (2018) 1362–1391. [15] P. Garg, S. Kumar, I. Choudhuri, A. Mahata, B. Pathak, Hexagonal planar CdS monolayer sheet for visible light photocatalysis, J. Phys. Chem C 120 (2016) 7052–7060. [16] X. Wu, J. Chen, C. Tan, Y. Zhu, Y. Han, H. Zhang, Controlled growth of high-density CdS and CdSe nanorod arrays on selective facets of two-dimensional semiconductor nanoplates, Nat. Chem. 8 (2016) 470–475.
[17] Y.F. Lin, J. Song, Y. Ding, S.Y. Lu, Z.L. Wang, Alternating the output of a CdS nanowire nanogenerator by a whiteǦ lightǦ stimulated optoelectronic effect, Adv. Mater. 20 (2008) 3127–3130. [18] R. Ranjan, M. Kumar, A.S.K. Sinha, Development and characterization of rGO supported CdS–MoS2 photoelectrochemical catalyst for splitting water by visible light, Int. J. Hydrogen Energy 44 (2019) 16176–16189. [19] S. Liu, Z.R. Tang, Y. Sun, J.C. Colmenares, Y.J. Xu, One-dimension-based spatially ordered architectures for solar energy conversion, Chem. Soc. Rev. 44 (2015) 5053–5075. [20] Z.R. Tang, Y. Zhang, Y.J. Xu, One dimensional CdS based materials for artificial photoredox reactions, ACS Appl. Mater. Interfaces 4 (2012) 1512–1520. [21] X.L. Yin, L.L. Li, D.C. Li, Z.J. Li, Y.X. Wang, X.J. Kong, J.S. Zhao, J.H. Jiang, J.C. Qian, D.H.
Pang,
X.X.
Du,
J.M.
Dou,
Noble-metal-free
CdS@MoS2
core-shell
nanoheterostructures for efficient and stabilized visible-light-driven H2 generation, Int. J. Hydrogen Energy 44 (2019) 16657–16666. [22] R. Ranjan, M. Kumar, A.S.K. Sinha, Development and characterization of rGO supported CdS–MoS2 photoelectrochemical catalyst for splitting water by visible light, Int. J. Hydrogen Energy 44 (2019) 16176–16189. [23] K. Kaviyarasu, E. Manikandan, M. Maaza, Synthesis of CdS flower-like hierarchical microspheres as electrode material for electrochemical performance, J. Alloys Compd. 648 (2015) 559–563. [24] J. Choi, D.A. Reddy, N.S. Han, S. Jeong, S. Hong, D.P. Kumar, J.K. Song, T.K. Kim, Modulation of charge carrier pathways in CdS nanospheres by integrating MoS2 and Ni2P for improved migration and separation toward enhanced photocatalytic hydrogen evolution, Catal. Sci. Technol. 7 (2017) 641–649. [25] K. Kombaiah, J.J. Vijaya, L.J. Kennedy, M. Bououdina, K. Kaviyarasu, R.J. Ramalingam, M.A. Munusamy, A. AlArfaj, Effect of Cd2+ concentration on ZnFe2O4 nanoparticles on the structural, optical and magnetic properties. Optik 135 (2017) 190–199. [26] H. Wang, W. Zhu, B. Chong, K. Qin, Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure, Int. J. Hydrogen Energy 39 (2014) 90–99.
[27] Q. Wang, S. Li, J. Qiao, R. Jin, Y. Yu, S. Gao, CdS–CdSe (CdTe) core–shell quantum dots sensitized TiO2 nanotube array solar cells, Sol. Energy. Mater. Sol. Cells 132 (2015) 650– 654. [28] Y. Zhu, R. Wang, W. Zhang, H. Ge, L. Li, CdS and PbS nanoparticles co-sensitized TiO2 nanotube arrays and their enhanced photoelectrochemical property, Appl. Surf. Sci. 315 (2014) 149–153. [29] T. Di, B. Cheng, W. Ho, J. Yu, H. Tang, Hierarchically CdS–Ag2S nanocomposites for efficient photocatalytic H2 production, Appl. Surf. Sci. 470 (2019) 196–204. [30] S. Wang, B. Zhu, M. Liu, L. Zhang, J. Yu, M. Zhou, Direct Z-scheme ZnO/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Appl. Catal. B 243 (2019) 19–26. [31] T. Di, B. Zhu, J. Zhang, B. Cheng, J. Yu, Enhanced photocatalytic H2 production on CdS nanorod using cobalt-phosphate as oxidation cocatalyst, Appl. Surf. Sci. 389 (2016) 775– 782. [32] S. Hong, D.P. Kumar, E.H. Kim, H. Park, M. Gopannagari, D.A. Reddy, T.K. Kim, Earth abundant transition metal-doped few-layered MoS2 nanosheets on CdS nanorods for ultraefficient photocatalytic hydrogen production, J. Mater. Chem A. 5 (2017) 20851–20859. [33] M. Gopannagari, D.P. Kumar, D.A. Reddy, S. Hong, M.I. Song, T.K. Kim, In situ preparation of few-layered WS2 nanosheets and exfoliation into bilayers on CdS nanorods for ultrafast charge carrier migrations toward enhanced photocatalytic hydrogen production. J. Catal. 351 (2017) 153–160. [34] B. Chai, M. Xu, J. Yan, Z. Ren, Remarkably enhanced photocatalytic hydrogen evolution over MoS2 nanosheets loaded on uniform CdS nanospheres, Appl. Surf. Sci, 430 (2018) 523–530. [35] R.K. Chava, J. Yeon, M. Kang, Hydrothermal growth of two dimensional hierarchical MoS2 nanospheres on one dimensional CdS nanorods for high performance and stable visible photocatalytic H2 evolution, Appl. Surf. Sci. 433 (2018) 240–248. [36] J. Li, Y. Peng, X. Qian, J. Lin, Few-layer Co-doped MoS2 nanosheets with rich active sites as an efficient cocatalyst for photocatalytic H2 production over CdS, Appl. Surf. Sci. 452 (2018) 437–442.
[37] Z.W. Zhang, Q.H. Li, X.Q. Qiao, D. Hou, D.S. Li, One-pot hydrothermal synthesis of willow branch-shaped MoS2/CdS heterojunctions for photocatalytic H2 production under visible light irradiation, Chinese J. Catal. 40 (2019) 371–379. [38] J. Wan, R. Wang, L. Liu, J. Fan, E. Liu, X. Gao, F. Fu, A novel approach for high-yield solid few-layer MoS2 nanosheets with effective photocatalytic hydrogen evolution, Int. J. Hydrogen Energy 44 (2019) 16639–16647. [39] X. Jiang, Y. Song, M. Dou, J. Ji, F. Wang, Selective growth of vertically aligned twodimensional MoS2/WS2 nanosheets with decoration of Bi2S3 nanorods by microwaveassisted hydrothermal synthesis: Enhanced photo-and electrochemical performance for hydrogen evolution reaction, Int. J. Hydrogen Energy 43 (2018) 21290–21298. [40] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, MoS2 nanoparticles grown on graphene: An advanced catalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 133 (2011) 7296–7299. [41] D.J. Late, B. Liu, H.R. Matte, V.P. Dravid, C.N.R. Rao, Hysteresis in single-layer MoS2 field effect transistors, ACS Nano 6 (2012) 5635–5641. [42] B. Radisavljevic, A. Radenovic, J. Brivio, I.V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [43] Z. Wang, Z. Jin, G. Wang, B. Ma, Efficient hydrogen production over MOFs (ZIF-67) and g-C3N4 boosted with MoS2 nanoparticles, Int. J. Hydrogen Energy 43 (2018) 13039–13050. [44] O. Khaselev, J.A. Turner, A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting, Science 280 (1998) 425–427. [45] L.Q. Jing, Y.G. Xu, Z.G. Chen, M.Q. He, M. Xie, J. Liu, H. Xu, S.Q. Huang, H.M. Li, Different morphologies of SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation, ACS Sustainable. Chem. Eng. 6 (2018) 5132–5141. [46] P.
Sinsermsuksakul, L.
Sun, S.W.
Lee, H.H.
Park, S.B.
Kim, C.
Yang, R.G.
Gordon, Overcoming efficiency limitations of SnS2Ǧ based solar cells, Adv. Energy Mater. 4 (2014) 1400496.
[47] H.W. Chen, Y.T. Chen, H. Zhang, D.W. Zhang, P. Zhou, J. Huang, Suspended SnS2 layers by light assistance for ultrasensitive ammonia detection at room temperature, Adv. Funct. Mater. 28 (2018) 1801035.
[48] W.P. Sun, X.H. Rui, D. Yang, Z.Q. Sun, B. Li, W.Y. Zhang, Y. Zong, S. Madhavi, S. Dou, Q.Y. Yan, Two-dimensional tin disulfide nanosheets for enhanced sodium storage, ACS Nano 9 (2015) 11371–11381. [49] Y. Liu, Y. Yang, X. Wang, Y. Dong, Y. Tang, Z. Yu, Z. Zhao, J. Qiu, Flexible paper like free standing electrodes by anchoring ultrafine SnS2 nanocrystals on graphene nanoribbons for high-performance sodium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 15484– 15491. [50] M.A. Lukowski, A.S. Daniel, C.R. English, F. Meng, A. Forticaux, R.J. Hamers, S. Jin, Highly active hydrogen evolution catalysis from metallic WS2 nanosheets, Energy Environ. Sci. 7 (2014) 2608–2613. [51] D.L. Greenaway, R. Nitsche, Preparation and optical properties of group IV–VI2 chalcogenides having the CdI2 structure, J. Phys. Chem. Solids 26 (1965) 1445–14458. [52] Z. Zhang, C. Shao, X. Li, Y. Sun, M. Zhang, J. Mu, Y. Liu, Hierarchical assembly of ultrathin hexagonal SnS2 nanosheets onto electrospun TiO2 nanofibers: Enhanced photocatalytic activity based on photoinduced interfacial charge transfer, Nanoscale 5 (2013) 606–618. [53] Z. Zhang, J. Huang, M. Zhang, Q. Yuan, B. Dong, Ultrathin hexagonal SnS2 nanosheets coupled with g-C3N4 nanosheets as 2D/2D heterojunction photocatalyts toward high photocatalytic activity, Appl. Catal. B 163 (2015) 298–305. [54] J. Low, J. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [55] Q. Xu, L. Zhang, J. Yu, S. wageh, A. A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: Principles, synthesis, and applications, Mater. Today 21 (2018) 1042–1063. [56] Y. Huo, Y. Yang, K. Dai, J. Zhang, Construction of 2D/2D porous graphitic C3N4/SnS2 composite as a direct Z-scheme system for efficient visible photocatalytic activity, Appl. Surf. Sci. 481 (2019) 1260–1269. [57] Y. Huo, J. Zhang, K. Dai, Q. Li, J. Lv, G. Zhu, C. Liang, All-solid-state artificial Z-scheme porous g-C3N4/Sn2S3-DETA heterostructure photocatalyst with enhanced performance in photocatalytic CO2 reduction, Appl. Catal. B 241 (2019) 528–538. [58] T. Hu, P. Li, J. Zhang, C. Liang, K. Dai, Highly efficient direct Z-scheme WO3/CdSdiethylenetriamine photocatalyst and its enhanced photocatalytic H2 evolution under visible light irradiation, Appl. Surf. Sci. 442 (2018) 20–29.
[59] J. Low, B. Dai, T. Tong, C. Jiang, J. Yu, In situ irradiated XǦ ray photoelectron spectroscopy investigation on a direct ZǦ Scheme TiO2/CdS composite film photocatalyst, Adv. Mater. 31 (2018) 1802981. [60] E. Huang, X. Yao, W. Wang, G. Wu, N. Guan, L. Li, SnS2 nanoplates with specific facets exposed for enhanced visibleǦ lightǦ driven photocatalysis, ChemPhotoChem 1 (2017) 60– 69. [61] K. Zhang, M. Fujitsuka, Y. Du, T. Majima, 2D/2D heterostructured CdS/WS2 with efficient charge separation improving H2 evolution under visible light irradiation, ACS Appl. Mater. Interfaces 10 (2018) 20458–20466. [62] K. Kaviyarasu, P.A. Devarajan, S.S.J. Xavier, S.A. Thomas, S. Selvakumar, One pot synthesis and characterization of cesium doped SnO2 nanocrystals via a hydrothermal process, J. Mater. Sci. Technol. 28 (2012) 15–20. [63] K. Kaviyarasu, E. Manikandan, J. Kennedy, M. Jayachandran, R. Ladchumananandasiivam, U.U. De Gomes, M. Maaza, Synthesis and characterization studies of NiO nanorods for enhancing solar cell efficiency using photon upconversion materials, Ceramics Int. 42 (2016) 8385–8394. [64] J. Yu, J. Jin, B. Cheng, M. Jaroniec, A noble metal-free reduced graphene oxide–CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel, J. Mater. Chem. A. 2 (2014) 3407–3416. [65] Q. Xiang, B. Cheng, J. Yu, GrapheneǦ based photocatalysts for solarǦ fuel generation, Angew. Chem. Int. Ed. 54 (2015) 11350–11366. [66] C.M. Magdalane, K. Kaviyarasu, A. Raja, M.V. Arularasu, G.T. Mola, A.B. Isaev, N.A. AlDhabi, M.V. Arasu, B. Jeyaraj, J. Kennedy, M. Maaza, Photocatalytic decomposition Photocatalytic decomposition effect of erbium doped cerium oxide nanostructures driven by visible light irradiation: Investigation of cytotoxicity, antibacterial growth inhibition using catalyst, J. Photochem. Photobiol. B 185 (2018) 275–282. [67] A.A. Ezhilarasi, J.J. Vijaya, K. Kaviyarasu, L.J. Kennedy, R.J. Ramalingam, H.A. AlLohedan, Green synthesis of NiO nanoparticles using Aegle marmelos leaf extract for the evaluation of in-vitro cytotoxicity, antibacterial and photocatalytic properties, J. Photochem. Photobiol. B 180 (2018) 39–50.
[68] S.K. Jesudoss, J.J. Vijaya, P.I. Rajan, K. Kaviyarasu, M. Sivachidambaram, L.J. Kennedy, H.A. Al-Lohedan, R. Jothiramalingam, M.A. Munusamy, High performance multifunctional green Co3O4 spinel nanoparticles: Photodegradation of textile dye effluents, catalytic hydrogenation of nitro-aromatics and antibacterial potential, Photochem. Photobiol. Sci. 16 (2017) 766–778. [69] W. Zhang, Y. Wang, Z. Wang, Z. Zhong, R. Xu, Highly efficient and noble metal-free NiS/CdS photocatalysts for H2 evolution from lactic acid sacrificial solution under visible light, Chem. Commun. 46 (2010) 7631–7633.
SCHEMES AND FIGURES
Scheme 1. Preparations of CdS nanorods, SnS2 nanosheets and CdS/SnS2 nanocomposites.
100
200
300
400
500
Intensity (a. u.)
Cd 3p3 Cd 3p1
Sn 3d O 1s
C 1s
S 2s
Sn 4d Sn 4p Sn 4s S 2p
Intensity (a. u.) 0
S2p3/2
(b) S 2p Cd 3d
(a)
600
700
800
S2p1/2
156 157 158 159 160 161 162 163 164 165 166
Binding energy (eV)
(d) Cd 3d
Sn 3d3/2
Intensity (a. u.)
Sn 3d5/2
Intensity (a. u.)
(c) Sn 3d
Binding energy (eV)
Cd3d5/2 Cd3d3/2
480 482 484 486 488 490 492 494 496 498 500
400 402 404 406 408 410 412 414 416 418
Binding energy (eV)
Binding energy (eV)
Fig. 1. X-ray photoelectron spectra of CdS/SnS2 nanocomposites: (a) Survey spectrum and (b)–(d) expanded spectra of S 2p, Sn 3d and Cd 3d regions, respectively.
Fig. 2. (a) TEM image, (b, c) HRTEM images and (d)–(i) elemental analyses of Cd, Sn and S elements, respectively, of CdS/SnS2 nanocomposites.
Fig. 3. (a) UV–visible diffuse reflectance spectra, (b) PL spectra, (c) PEC analyses and (d) electrochemical impedance spectra of CdS nanorods and CdS/SnS2 nanocomposites.
(b)
35 (c) -1
Amount of H2 (mmol. h )
3rd Reproducibility
-1
-1
Amount of H2 (mmol. g . h )
(a) 20 15 10 5
2nd
1st
0 2
3
5
6
7
10
0
20 15 10 5
10
Lactic acid addition
20
0.112
0 0
1
2
3
4
5
Lactic acid (mL)
-1
15 10 5
3
4
5
6
Catalyst dosage (mg)
Amount of H2 (mmol. g )
-1
10
2
-1
-1
15
1
20
500
(e) Amount of H2 (mmol. g . h )
20 (d) -1
15
Amount of H2 (mmol. g . h )
2
-1
5
-1
Amount of SnS loading on CdS (wt%)
Amount of H2 (mmol. g . h )
25
0 0
5
30
(f)
400 Ar purged
300 200 100 0
0 1st
2nd
3rd
4th
Number of cycles
5th
0
5
10
15
20
25
30
35
Irradiation time (h)
Fig. 4. Photocatalytic assessments of optimized CdS/SnS2 nanocomposites: (a) effect of SnS2 loading wt. onto CdS nanorods, (b) reproducibility tests, (c) effects of the catalyst dosage and (d) scavenger effect on photocatalytic performance of the CdS/SnS2 nanocomposites, (e) recyclability of CdS/SnS2 nanocomposite and (f) long-term stability of the CdS/SnS2 nanocomposite under continuous photo-irradiation.
Scheme 2. Proposed photocatalytic H2 production mechanism of CdS/SnS2 nanocomposites with band-gap structures under simulated solar irradiation.
!"#$%&%'()*+),+(*'"&"-'-+ ! !*!"#$!%&'#()*!+$,-%)$!'#%'!'#$.!#%/$!0(!10(20!,(34$'506!750%0,5%-!50'$)$*'*!()!4$)*(0%-!)$-%'5(0*#54܈ !'#%'!,(&-+!#%/$!%44$%)$+!'(!507-&$0,$!'#$!2()1!)$4()'$+!50!'#5*!4%4$)8 ! !!"#$!%&'#()*!+$,-%)$!'#$!7(--(2506!750%0,5%-!50'$)$*'*94$)*(0%-!)$-%'5(0*#54*!2#5,#!3%.!:$!,(0*5+$)$+ !!;*'*%*!4('$0'5%-!,(34$'506!50'$)$ ! !
! ! ! !
!"#$%&'(%)#&*+"#*%)',#-#./.)#' !0' 1"##-' 2-)3-44-!" #$%&'()*+,-.+)-$%/" 0$12+," +%+,34-4/" 5%6'4)-7+)-$%/" 81-)-%7" 9" :1-7-%+," ;1+<)" 50'1&-6..)'7"/-&!"#$%&'()*+,-.+)-$%/"0$12+,"+%+,34-4/"5%6'4)-7+)-$%/"81-)-%7"9"='6-'>"+%?" @?-)-%7" 8-9$":"9-)-';%4-))-3-&*!"0$12+,"+%+,34-4/"A+,-?+)-$%" 50'!/-&-)-#$-'2.99'?%)3!"A+,-?+)-$%/"A-4*+,-.+)-$%" ="@*)'7*/!"A+,-?+)-$%/"A-4*+,-.+)-$%" A-.' 7<"' 7*/!" #$%&'()*+,-.+)-$%/" B*('16-4-$%/" 81-)-%7" 9" ='6-'>" +%?" @?-)-%7/" 0*%?-%7" C&D*-4-)-$%" "
Graphical Abstract
·
Optimal hydrogen evolution rate of 20.2 mmol h−1 g−1 facilitated by enhanced absorption and prolonged life time of charge carriers by SnS2 as co-catalyst on CdS.
!"#"$%&'()*+',*+'-#( ( !! !"#$%&'%()*+%',$-($.'/"0&'-(%10$(2)&3)(&4(-&-$(5'/+4()&,)$%(0)1-(0)$(6+!7( !! 893$//$-0(4$#1%10&'-('5(#)'0'(,$-$%10$+(3)1%,$(31%%&$%4('-(6+!:!-!;()*<%&+(2&0)(!-!;( &-4$%0&'-7( !! 893$//$-0(#)'0'(401<&/&0*('5(0)$(3101/*40(5'%(/'-,(0$%=(#%130&31/(1##/&310&'-4(%$=1&-$+( $55$30&.$(5'%(>?()("-+$%('#0&=&@$+(3'-+&0&'-47( !! A-&5'%=(+$#'4&0&'-('5("/0%10)&-(!-!;(4)$$04('-0'(6+!(<*(4&=#/$(=$0)'+47(