Construction of High-Quality SnO2@MoS2 Nanohybrids for Promising

Mar 1, 2017 - ... Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, Xi'an Jiaotong University, Xi'an 710049...
3 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/IC

Construction of High-Quality SnO2@MoS2 Nanohybrids for Promising Photoelectrocatalytic Applications Xinyu Zhang,† Yawei Yang,‡ Shujiang Ding,§ Wenxiu Que,‡ Zhiping Zheng,∥ and Yaping Du*,† †

Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, P. R. China ‡ Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, P. R. China § State Key Laboratory for Mechanical Behavior of Materials, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, and Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, P. R. China ∥ Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0041, United States S Supporting Information *

ABSTRACT: High-quality three-dimensional (3D) hierarchical SnO2@MoS2 nanohybrids were successfully obtained via a facile but effective wet chemistry synthesis method. Meanwhile, the SnO2@MoS2 hybrid film was fabricated through an electrophoretic deposition method to promote photoelectrocatalytic (PEC) efficiency and solve the recovery problem. Compared with the pure SnO2 and MoS2 films, the SnO2@ MoS2 heterostructures could decrease the rate of the photoelectron−hole pair’s recombination, which resulted in the superior PEC pollutant degradation and water splitting activities. Meanwhile, the SnO2@MoS2 hybrid films with welldefined 3D hierarchical configurations have large surface areas, abundant active edge sites, and defects on the basal surfaces, which were also advantageous for the PEC activities (for pollutant degradation, apparent rate constant k = 5.91 h−1; for water splitting, onset potential = −0.05 V and current density = 10 mA/cm2). Therefore, the SnO2@MoS2 hybrid film proved to be a superior structure for PEC applications.

■

band gap semiconductors to build hybrid structures,7 and fabrication of recyclable films with superior optical, optoelectronic, and electronic properties.8 Among these strategies, growth of hierarchical SnO2/metal sulfide heterostructures with large surface areas provides a facile, effective, and low-cost way to enhance the PEC activities through broadening the light harvesting window to the visible region and facilitating charge separation and transfer. To the best of our knowledge, as a common narrow bandgap material, MoS2 with a typical layered sandwiched structure, in which molybdenum atoms are sandwiched between two layers of sulfur atoms, exhibited high photon-to-current and photogenerated carrier efficiency and other unusual optical properties.9 Furthermore, MoS2 nanoflakes with a large surface area and abundant active edge sites exhibited superior photoelectricity-induced catalytic activities.10 Therefore, few-layer MoS2 nanoflakes were employed as a superior candidate to sensitize SnO2 to form a SnO2@MoS2 core−shell hierarchical hybrid structure, which was expected to enhance the PEC

INTRODUCTION Over the past several decades, photoelectrocatalytic (PEC) pollutant degradation and water splitting have attracted significant interest for the purpose of environmental protection and clean energy production.1 Transition metal oxides (TMOs) as photoanodes, with well-defined morphologies, have great potential in the application of PEC pollutant degradation and water splitting.2 Among various TMOs, tin oxide (SnO2) has received a great deal of attention as a photoanode because of its favorable energy level position, high photoactivity, excellent photocorrosion resistance and chemical stability, electron transport properties, and low cost.3 All of the properties are beneficial for catalysis and sensing.4 However, to further promote the performance of SnO2 in the field of catalysis, particularly for PEC applications, some barriers must be overcome. The first problem is its relatively large band gap, ∼3.5 eV, which limits its photoresponse to visible light; the second is fast recombination of electron−hole pairs in SnO2, and the third is the recycling problem of powder catalysts. To overcome these limitations, various strategies have been employed, such as doping with metal and non-metal ions,5 modification with noble metals,6 combination with narrow © 2017 American Chemical Society

Received: December 2, 2016 Published: March 1, 2017 3386

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry Scheme 1. Illustration of the Formation of SnO2@MoS2 Nanohybrids and Their PEC Application

properties. Meanwhile, semiconductor films with a large surface area can reduce the extent of light loss, increase the contact area with reactants,11 and solve the recovery problem.12 Hence, the SnO2@MoS2 heterostructures were further fabricated to a film as a photoanode to promote the PEC applications.13 In this work, we developed a facile but effective wet chemistry method to synthesize well-defined three-dimensional (3D) hierarchical SnO2@MoS2 nanohybrids (Scheme 1), which consist of a core of a SnO2 nanotube and a shell of few-layer MoS2 nanoflakes. Then, via the electrophoretic deposition (EPD) method, we prepared SnO2, MoS2, and SnO2@MoS2 films as photoanodes to demonstrate their PEC pollutant degradation and water splitting activities. Compared with pure SnO2 nanotubes and MoS2 nanoflakes, the formation of highquality SnO2@MoS2 heterostructures with a well-defined 3D hierarchical morphology could improve the visible light absorption efficiency, charge generation, and separation efficiency, thus yielding remarkably enhanced PEC activities.

■

oven. The pure MoS2 nanoflakes were synthesized via the same hydrothermal process in the absence of SnO2 nanotubes, as shown in Figure S1. Fabrication of SnO2, MoS2, and SnO2@MoS2 Films by the EPD Process. The SnO2@MoS2 hybrid film on fluorine-doped tin oxide (FTO, 14 Ω/square) glass was fabricated via the EPD method17 with a two-electrode system (FTO glass as the working electrode and Pt foil as the counter electrode). In a typical procedure, the asprepared SnO2@MoS2 nanohybrids were dispersed well in acetone at a concentration of 1 mg/mL (ζ potential of −13.96 mV). Then, a constant voltage of 100 V/cm was applied for 10 min. Afterward, the SnO2@MoS2 hybrid film was rinsed with acetone and then annealed at 500 °C for 1 h in an Ar atmosphere. Meanwhile, the SnO2 and MoS2 (ζ potentials of −3.56 and −8.24 mV, respectively) films were also obtained like that of SnO2@MoS2 was. The scanning electron microscopy (SEM) images and photographs of SnO2@MoS2, SnO2, and MoS2 films are shown in Figure 7 and Figure S3, respectively. Photoelectrocatalytic (PEC) Measurements. PEC experiments were conducted in a typical three-electrode system (sample films as the working electrodes, Pt foil as the counter electrode, and Ag/AgCl in a saturated KCl solution as the reference electrode) on an electrochemical workstation (CHI660E, Chenhua Instrument Co. Ltd.) under 150 W xenon lamp (400 nm cutoff filter, ∼300 mW/cm2) irradiation. For the photocurrent response measurement, photocurrent−time (I−t) curves were tested under a 0.50 V bias versus Ag/ AgCl in a 0.10 M Na2SO4 electrolyte solution. The PEC pollutant degradation performances were evaluated by degrading a rhodamine B (RhB) aqueous solution. Ten milliliters of a 5 mg/L RhB solution mixed with 10 mL of a 0.20 M Na2SO4 solution was used as the electrolyte. Afterward, the 0.50 V bias versus Ag/AgCl was employed to promote the charge separation and transfer. The concentration of the degraded RhB solution was recorded by the UV−vis absorption spectrum at 554 nm to assess degradation. For the water splitting test, linear sweep voltammetry (LSV) was conducted in 0.5 M H2SO4 with a scan rate of 2 mV/s. The Ag/AgCl electrode was employed with respect to the reversible hydrogen electrode (RHE): RHE (volts) = V vs Ag/AgCl (volts) + 0.197 (volt) + 0.0591 × pH.

EXPERIMENTAL SECTION

Materials. Sodium molybdate (Na2MoO4·2H2O, >99.90%, SigmaAldrich), thioacetamide (TAA, CH3CSNH2, >99.90%, Sigma-Aldrich), polymeric nanotubes (PNTs were prepared according to a previously reported method14), stannous chloride (SnCl2·2H2O, >99.90%, SigmaAldrich), urea (H2NCONH2, >99.99%, Sigma-Aldrich), mercaptoacetic acid (C2H4O2S, >95%, Sigma-Aldrich), hydrochloric acid (HCl, 37 wt %, Sigma-Aldrich), and sulfuric acid (H2SO4, 98 wt %, SigmaAldrich) were used. Synthesis of SnO2 Nanotubes. The SnO2@PNT nanotubes were prepared via a previously reported method.15 In a typical procedure, at room temperature, with vigorous stirring, in a 100 mL round-bottom flask, 0.10 g of acid-treated PNTs was dispersed in 40 mL of a 20 mM mercaptoacetic acid solution, and then under vigorous stirring, 0.50 mL of a 37 wt % HCl solution, 0.10 g of SnCl2·2H2O, and 0.50 g of urea were added to the solution one by one. After that, the temperature was increased to 60 °C for 6 h. After being cooled to room temperature, the as-obtained pure SnO2@PNTs nanotubes were separated by centrifugation (8000 rpm for 5 min) and washed with ethanol before being dried at 60 °C in a vacuum oven. Subsequently, the product was calcined for 2 h at 450 °C in air, and the welldeveloped mesoporous SnO2 nanotubes were successfully obtained. Synthesis of SnO2@MoS2 Nanohybrids. The SnO2@MoS2 nanohybrids were prepared via a hydrothermal method.16 In a typical procedure, 7 mg of Na2MoO4·2H2O and 15 mg of TAA were added to 13 mL of pure water that was being vigorously magnetically stirred for 45 min. After that, 7 mg of SnO2 was added to the solution; afterward, the resultant homogeneous solution was transferred into a 25 mL Teflon-lined autoclave. It was sealed, and its contents reacted at 180 °C for 18 h. After being cooled, the as-formed SnO2@MoS2 nanohybrids were collected by centrifugation (8000 rpm for 5 min) and washed with ethanol before being dried at 60 °C in a vacuum

■

RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) patterns of the assynthesized SnO2@MoS2 nanohybrids, pure SnO2 nanotubes, and pure MoS2 nanoflakes. In Figure 1, the observable XRD peaks of pure SnO2 nanotubes could be readily assigned to the tetragonal phase of SnO2 (cassiterite, JCPDS Card No. 411445, lattice constants a = b = 4.74 Å and c = 3.19 Å), and no diffraction peak from other chemical species, such as SnO, could be observed. As for the pure MoS2 nanoflakes, the crystal phase mainly consisted of hexagonal MoS2 (molybdenite-2H, JCPDS Card No. 37-1492, lattice constants a = b = 3.16 Å and c = 12.30 Å). The diffraction peaks of MoS2 did not appear in the XRD pattern of SnO2@MoS2 nanohybrids, indicating that 3387

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

Figure 1. XRD patterns of as-prepared SnO2@MoS2 nanohybrids, pure SnO2 nanotubes, and pure MoS2 nanoflakes.

the coating of MoS2 nanoflakes on the SnO2 nanotubes might consist of a few layers, which were too thin to be detected.16,18 The SEM and transmission electron microscopy (TEM) images of the as-synthesized pure SnO2 nanotubes (∼100 nm in width and >1 μm in length) are shown in panels a and b, respectively, of Figure 2. The pure MoS2 nanoflakes obtained via the same hydrothermal method in the absence of SnO2 nanotubes are shown in Figure S1. Meanwhile, the morphologies of as-obtained SnO2@MoS2 nanohybrids were further studied by SEM, HAADF-STEM, TEM, and HRTEM (Figure 2c−f). Figure 2c shows the SEM image of SnO2@MoS2 nanohybrids with a well-defined 3D hierarchical morphology. As seen from Figure 2d, the average thickness of SnO2 nanotubes was ∼100 nm and the diameter of the SnO2@ MoS2 nanohybrids was ∼300 nm; numerous thin MoS2 nanoflakes covered the surface of SnO2 nanotubes. Figure 2e shows the typical TEM image of SnO2@MoS2 nanohybrids, which demonstrated the MoS2 nanoflakes covered the surface of SnO2 nanotubes. The inset in Figure 2e illustrates the good crystallization of nanohybrids. The crystal lattice fringes of SnO2@MoS2 nanohybrids were clearly visible with a spacing of ∼0.33 nm, matching the spacing of (110) planes of tetragonal SnO2 (Figure 2f). Simultaneously, the inset of Figure 2f illustrates the lattice fringes with spacings of ∼0.27 and ∼0.64 nm, corresponding to the (100) and (002) planes of hexagonal MoS2, respectively. Nitrogen adsorption−desorption isotherm measurements were adopted to investigate the structures of the as-obtained SnO2@MoS2, MoS2, and SnO2 nanocrystals (Figure S2). The specific surface areas were calculated on the basis of the BET method.19 As shown in Figure S2, the BET surface areas could be estimated to be 93.22 m2/g for SnO2@MoS2 nanohybrids (Figure S2a), 84.29 m2/g for pure MoS2 nanoflakes (Figure S2b), and 66.37 m2/g for pure SnO2 nanotubes (Figure S2c). To study the elemental composition of SnO2 @MoS 2 nanohybrids, EDX mapping analysis was conducted (Figure 3). Figure 3 shows four different elements (Sn, Mo, O, and S) that exist in the nanohybrids; one can see that the inner tube core consists of Sn and O element and the shell consists of Mo and S elements. All of the results further confirmed MoS2 nanoflakes covered the surface of SnO2 nanotubes, resulting in SnO2@MoS2 nanohybrids.

Figure 2. (a) SEM and (b) TEM images of pure SnO2 nanotubes. (c) SEM, (d) HAADF-STEM, (e) TEM, and (f) HRTEM images of SnO2@MoS2 nanohybrids. The inset photograph of panel e shows the SAED patterns of the SnO2@MoS2 nanohybrid (the highlighted yellow area); the inset photograph of panel f shows the HRTEM image of MoS2 nanoflakes from the SnO2@MoS2 nanohybrid.

To study the chemical state of Sn, Mo, S, and O in the SnO2@MoS2 nanohybrids, XPS analysis was performed (Figure 4). Figure 4a depicts the XPS spectrum taken from the Sn 3d region of SnO2@MoS2 nanohybrids. The double peaks at 495.70 and 487.30 eV were attributed to the core levels of Sn 3d3/2 and Sn 3d5/2, respectively, indicating the Sn chemical state was positively tetravalent.20 The binding energies of Mo 3d3/2 and Mo 3d5/2 were located at 231.90 and 228.70 eV, respectively (Figure 4b), which suggested the main Mo chemical state was positively tetravalent. Meanwhile, a small proportion of MoS2 had been oxidized to MoO3 (Figure 4b).21 As shown in Figure 4c, the peaks at 162.90 and 161.90 eV were assigned to the binding energies of S 2p1/2 and S 2p3/2, respectively, demonstrating the S chemical state was negatively divalent.22 The O 1s peak at 531.60 eV shown in Figure 4d indicates the presence of crystal lattice oxygen in SnO2@MoS2 nanohybrids (Figure 4d).23 The optical properties of SnO2@MoS2 nanohybrids were characterized by UV−vis absorption spectroscopy (Figure 5). The pure SnO2 nanotubes exhibited absorption at ∼300 nm, which was a characteristic of intrinsic bandgap absorption. Compared to pure SnO2, SnO2@MoS2 nanohybrids exhibited enhanced absorption in the UV and visible light region. 3388

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

Figure 3. (a) TEM image of the SnO2@MoS2 nanohybrid. (b−e) EDX elemental mapping images of Sn, O, Mo, and S, respectively, in the selected area.

Figure 4. XPS spectra of SnO2@MoS2 nanohybrids: (a) Sn 3d, (b) Mo 3d, (c) S 2p, and (d) O 1s.

SnO2@MoS2 nanohybrids at ∼470 and 630 nm were exhibited.16

Notably, because of the strong quantum confinement effect of thin MoS2 nanoflakes, slight red-shifts of absorption peaks of 3389

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

Figure 5. UV−vis absorption spectra of SnO2@MoS2 nanohybrids, pure SnO2 nanotubes, and pure MoS2 nanoflakes.

Figure 7. Photocurrent responses of SnO2@MoS2 hybrid, MoS2, and SnO2 films at a 0.5 V bias vs Ag/AgCl.

SEM images of the fabricated SnO2@MoS2, SnO2, and MoS2 films are shown in Figure 6 and Figure S3, respectively. As shown in Figure 6, the SnO2@MoS2 nanohybrids were effectively deposited on the surface of the FTO glass by the EPD process, and the well-defined 3D hierarchical nanostructures still existed after the deposition, indicating that hybrid nanostructures could be maintained. The digital photo of the SnO2@MoS2 hybrid film is shown as an inset in Figure 6b, which confirmed the corresponding film. As shown in Figure S3, SnO2 and MoS2 films were obtained via the same method. It could be seen that the morphologies of SnO2 and MoS2 nanostructures were also well-maintained. The digital photos demonstrated the well-defined characteristics of SnO2 and MoS2 films (insets of Figure S3a,b). All of the results suggested that the deposition process could not damage the morphologies of nanostructures, and the hybrid film was successfully obtained. The photocurrent responses were measured under visible light irradiation to investigate the charge generation and separation efficiency of the photoinduced electron−hole pairs. The I−t curves of sample films are shown in Figure 7. One can see that the photocurrent density of the SnO2@MoS2 hybrid film was much higher than that of SnO2 and MoS2 films. It is

inferred that higher photocurrent means more photogenerated electrons. Meanwhile, the photocurrent decay was related to the electron lifetime or recombination rate. When the light was off, the photocurrents of SnO2 and MoS2 films rapidly decayed to a steady state. For the SnO2@MoS2 hybrid film, the photocurrent decayed gradually, indicating the relatively lower recombination rate of the photoinduced carrier. The increased photocurrent and decreased photocurrent decay suggested an improved charge generation and separation efficiency of the photoinduced electron−hole pairs for the hybrid film, which was promising for PEC applications. PEC degradation of the RhB solution was conducted by introducing the as-prepared film as a catalyst under visible light illumination and an anodic bias of 0.5 V versus Ag/AgCl. For comparison, photocatalytic (PC) degradation was also assessed. In Figure 8a, RhB was stable under visible light irradiation. For the PC process, ∼20, ∼50, and ∼85% of the RhB were photodegraded by using SnO2, MoS2, and SnO2@MoS2 films, respectively, after irradiation for 4 h. Among them, the SnO2@ MoS2 hybrid film showed the best PC activities. Compared with PC results, the PEC activities of the films were significantly improved because of the enhanced charge separation and transfer efficiency caused by the additional

Figure 6. SEM images of SnO2@MoS2 hybrid film (a) with low magnification and (b) with high magnification. The inset in panel b is the corresponding photograph of the SnO2@MoS2 hybrid film. 3390

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

Figure 8. (a) PC and PEC degradation. (b) Linear fitting of pseudo-first-order kinetics of RhB degradation. (c) Recycled PEC degradation test of the SnO2@MoS2 hybrid film.

and 5.91 × 10−3 h−1, respectively, which were almost 1.82, 1.73, and 1.26 times higher than that of the PC process, respectively. Moreover, the PEC k value of the SnO2@MoS2 hybrid film was almost 6 and 2 times higher than those of SnO2 and MoS2 films, respectively. The stable and recyclable performance of the SnO2@MoS2 hybrid film was conducted by repeating the PEC degradation five times. Figure 8c shows that the SnO2@MoS2 hybrid film maintained PEC activities after five cycles, suggesting the SnO2@MoS2 hybrid film attached firmly to the FTO substrate (Figure S4). The water splitting performance was also conducted to demonstrate the PEC activities of the hybrid film. LSV was performed both in the dark (only electrocatalysis condition) and under visible light illumination (PEC condition). A higher photocurrent corresponded to a higher efficiency of the PEC device for water splitting. Under the dark condition (Figure 9a), all samples showed negligible current densities. The SnO2@MoS2 hybrid film exhibited a higher water splitting activity with an onset potential of −0.1 V and a cathodic current density of 6 mA/cm2. In contrast, Figure 9b shows the LSV curves of SnO2@MoS2, SnO2, and MoS2 films under visible light illumination. Among them, the SnO2@MoS2 hybrid film revealed a water splitting activity (onset potential of −0.05 V and a sharp rise in current density to 10 mA/cm2) much higher than that under the dark condition, which confirmed an

applied potential. The remarkable enhanced PEC degradation results of ∼33, ∼69, and ∼97% were obtained for SnO2, MoS2, and SnO2@MoS2 films, respectively. The results demonstrate that an apparent synergetic effect between photo- and electrocatalysis existed in the PEC process. A linearity of degradation data could be established for all samples as demonstrated in Figure 8b, suggesting the pseudo-first-order kinetic reaction: ln(C0/C) = kt

(1)

where C, C0, and k are the RhB concentration at time t and zero and the apparent degradation rate constant, respectively. The fitted k values for all samples are listed in Table 1. For the PC Table 1. Values of Apparent Rate Constant k for Different Catalysts catalyst

PC k (×10−3 h−1)

PEC k (×10−3 h−1)

SnO2 MoS2 SnO2@MoS2

0.54 1.72 4.71

0.99 2.98 5.91

reactions, the k values of SnO2, MoS2, and SnO2@MoS2 films were 0.54, 1.72, and 4.71 × 10−3 h−1, respectively, while for the PEC process, the k values significantly increased to 0.99, 2.98,

Figure 9. LSV plots of SnO2@MoS2, SnO2, and MoS2 film electrodes (a) under the dark condition and (b) under visible light. 3391

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

Figure 10. Schematic illustration of the energy band structure and the PEC degradation process of the SnO2@MoS2 film under visible light.

Through the EPD process, SnO2@MoS2, SnO2, and MoS2 films were obtained. Compared with those of pure SnO2 and MoS2 films, the SnO2@MoS2 hybrid film showed enhanced PEC activities of pollutant degradation (k = 5.91 h−1) and water splitting (onset potential of −0.05 V and current density of 10 mA/cm2). The enhanced PEC activities of the SnO2@MoS2 hybrid film were mainly ascribed to (a) the formation of highquality SnO2@MoS2 heterostructures, which improved the absorption efficiency of visible light and inhibited the recombination of the photogenerated electron−hole pairs, (b) the well-defined 3D hierarchical morphology, which provided a large surface area, abundant active edge sites, and defects on the basal surfaces, and (c) the apparent synergetic effect between photo- and electrocatalysis, which facilitated charge generation, separation, and transfer. This study is expected to aid in the development of new hybrid material systems with advanced nanostructures and their films for superior and recyclable PEC applications.

apparent synergetic effect between photo- and electrocatalysis in the water splitting process. Meanwhile, the SnO2@MoS2 hybrids with 3D hierarchical nanostructures possessed larger surface areas, abundant active edge sites, and defects on the surface, which showed the best water splitting performance. From the results presented above, the PEC activities of SnO2 nanotubes could be greatly improved after hybridization with MoS2 nanoflakes, and the synergetic performance of photo- and electrocatalysis was better than that of photocatalysis alone in RhB degradation and electrocatalysis alone in water splitting. Herein, possible mechanisms of the enhanced PEC activities for the SnO2@MoS2 hybrid film are proposed. In the PEC degradation process (Figure 10), under light irradiation, photogenerated electrons were instantaneously excited from the valence band (VB) of MoS2 to the conduction band (CB) and then rapidly transferred to the CB of SnO2 because of the well-matched energy bands between MoS2 and SnO2.16,18 The MoS2 expanded the absorption through the whole visible light region, resulting in more carrier generation. Meanwhile, recombination of the electron−hole pairs was inhibited by the heterostructures. The highly oxidative hydroxyl radicals (OH•) that originated from the photoinduced holes could oxidize organics into minerals. Simultaneously, the photoinduced electrons were rapidly transferred to the counter electrode through the FTO layer and outer circuit, which were captured by O2 in the aqueous solution to produce superoxide radical (O2 + e− → O2•−) and then degraded RhB into inorganic products. The process integrated with the bias potential and illumination took full advantage of visible light and facilitated charge generation, separation, and transfer. In the PEC water splitting process, similar to that of PEC degradation, the matched energy band of SnO2@MoS2 nanohybrids favored charge transfer and decreased the rate of photoelectron−hole recombination between MoS2 and SnO2, which prolonged the lifetime of the electron−hole pairs and led to the superior water splitting activities; meanwhile, the hybrids with well-defined 3D hierarchical nanostructures provided large surface areas, abundant active edge sites, and defects on the basal surfaces, which were also advantageous for the PEC activities.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02914. TEM image of pure MoS2 nanoflakes, SEM images of SnO2 and MoS2 films, SEM image of the SnO2@MoS2 hybrid film after PEC degradation, and a schematic illustration of the energy band structure and the PEC degradation process of the SnO2@MoS2 film under visible light (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yaping Du: 0000-0002-9937-2087 Author Contributions

X.Z. and Y.Y. contributed equally to this work. Notes

■

The authors declare no competing financial interest.

■

CONCLUSION In summary, the 3D hierarchical SnO2@MoS2 nanohybrids were prepared for the first time via a wet chemistry method by using SnO2 nanotubes with a rough surface as the template.

ACKNOWLEDGMENTS We gratefully acknowledge the financial aid from the start-up funding from Xi’an Jiaotong University, the Fundamental 3392

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393

Article

Inorganic Chemistry

(15) Xu, X.; Liang, J.; Zhou, H.; Lv, D. M.; Liang, F. X.; Yang, Z. L.; Ding, S. J.; Yu, D. M. The preparation of uniform SnO2 nanotubes with a mesoporous shell for lithium storage. J. Mater. Chem. A 2013, 1, 2995−2998. (16) Zhou, W. J.; Yin, Z. Y.; Du, Y. P.; Huang, X.; Zeng, Z. Y.; Fan, Z. X.; Liu, H.; Wang, J. Y.; Zhang, H. Synthesis of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures for enhanced photocatalytic activities. Small 2013, 9, 140−147. (17) Yang, Y.; Que, W.; Zhang, X.; Xing, Y.; Yin, X.; Du, Y. Facile synthesis of ZnO/CuInS2 nanorod arrays for photocatalytic pollutants degradation. J. Hazard. Mater. 2016, 317, 430−439. (18) Chang, K.; Chen, W. X. Single-layer MoS2/Graphene dispersed in amorphous carbon: towards high electrochemical performances in rechargeable lithium ion batteries. J. Mater. Chem. 2011, 21, 17175− 17184. (19) Cai, R.; Chen, J.; Yang, D.; Zhang, Z. Y.; Peng, S. J.; Wu, J.; Zhang, W. Y.; Zhu, C. F.; Lim, T. M.; Zhang, H.; Yan, Q. Y. Solvothermal-induced conversion of one-dimensional multilayer nanotubes to two-dimensional hydrophilic VOx nanosheets: synthesis and water treatment application. ACS Appl. Mater. Interfaces 2013, 5, 10389−10394. (20) Szuber, J.; Czempik, G.; Larciprete, R.; Koziej, D.; Adamowicz, B. XPS study of the L-CVD deposited SnO2 thin films exposed to oxygen and hydrogen. Thin Solid Films 2001, 391, 198−203. (21) Wang, P. P.; Sun, H. Y.; Ji, Y. J.; Li, W. H.; Wang, X. Threedimensional assembly of single-layered MoS2. Adv. Mater. 2014, 26, 964−969. (22) Skeldon, P.; Wang, H. W.; Thompson, G. E. Formation and characterization of self-lubricating MoS2 precursor films on anodized aluminium. Wear 1997, 206, 187−196. (23) Zhang, X. Y.; Ge, J.; Lei, B.; Xue, Y. M.; Du, Y. P. High quality β-FeOOH nanostructures constructed by a biomolecule-assisted hydrothermal approach and their pH-responsive drug delivery behaviors. CrystEngComm 2015, 17, 4064−4069.

Research Funds for the Central Universities (2015qngz12), the China National Funds for Excellent Young Scientists (Grant 21522106), the National Science Foundation of China (Grant 21371140), and U.S. National Science Foundation (Grant CHE-1152609). We also thank Dr. Xinghua Li from Northwest University (Xi’an, China) for the HRTEM characterization.

■

REFERENCES

(1) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (2) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (3) Chen, J. S.; Lou, X. W. SnO2-based nanomaterials: synthesis and application in lithium-ion batteries. Small 2013, 9, 1877−1893. (4) Ansari, S. A.; Khan, M. M.; Omaish Ansari, M.; Lee, J.; Cho, M. H. Highly photoactive SnO2 nanostructures engineered by electrochemically active biofilm. New J. Chem. 2014, 38, 2462−2469. (5) Zhang, D. L.; Deng, Z. B.; Zhang, J. B.; Chen, L. Y. Microstructure and electrical properties of antimony-doped tin oxide thin film deposited by sol−gel process. Mater. Chem. Phys. 2006, 98, 353−357. (6) Mir, F. A.; Batoo, K. M. Effect of Ni and Au ion irradiations on structural and optical properties of nanocrystalline Sb-doped SnO2 thin films. Appl. Phys. A: Mater. Sci. Process. 2016, 122, 418−425. (7) Liu, Y.; Yu, Y. X.; Zhang, W. D. MoS2/CdS heterojunction with high photoelectrochemical activity for H2 evolution under visible light: the role of MoS2. J. Phys. Chem. C 2013, 117, 12949−12957. (8) Ji, J.; Guo, L. L.; Li, Q.; Wang, F.; Li, Z. L.; Liu, J. J.; Jia, Y. A bifunctional catalyst for hydrogen evolution reaction: the interactive influences between CdS and MoS2 on photoelectrochemical activity. Int. J. Hydrogen Energy 2015, 40, 3813−3821. (9) (a) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575− 6578. (b) Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H. Single-layer MoS 2 phototransistors. ACS Nano 2012, 6, 74−80. (10) (a) Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsøe, H.; Clausen, B. S.; Laegsgaard, E.; Besenbacher, F. Size-dependent structure of MoS2 nanocrystals. Nat. Nanotechnol. 2007, 2, 53−58. (b) Zhang, X.; Lai, Z. C.; Tan, C. L.; Zhang, H. Solution-processed two-dimensional MoS2 nanosheets: preparation, hybridization, and applications. Angew. Chem., Int. Ed. 2016, 55, 8816−8838. (c) Lu, Q. P.; Yu, Y. F.; Ma, Q. L.; Chen, B.; Zhang, H. 2D Transition-metaldichalcogenide-nanosheet-based composites for photocatalytic and electrocatalytic hydrogen evolution reactions. Adv. Mater. 2016, 28, 1917−1933. (d) Tan, C. L.; Zhang, H. Two-dimensional transition metal dichalcogenide nanosheet-based composites. Chem. Soc. Rev. 2015, 44, 2713−2731. (e) Chen, J. Z.; Wu, X. J.; Yin, L. S.; Li, B.; Hong, X.; Fan, Z. X.; Chen, B.; Xue, C.; Zhang, H. One-pot synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution. Angew. Chem., Int. Ed. 2015, 54, 1210−1214. (11) Yuan, Y. P.; Ruan, L. W.; Barber, J.; Joachim Loo, S. C.; Xue, C. Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion. Energy Environ. Sci. 2014, 7, 3934−3951. (12) Kuo, T. J.; Lin, C. N.; Kuo, C. L.; Huang, M. H. Growth of ultralong ZnO nanowires on silicon substrates by vapor transport and their use as recyclable photocatalysts. Chem. Mater. 2007, 19, 5143− 5147. (13) Khan, M. M.; Ansari, S. A.; Khan, M. E.; Ansari, M. O.; Min, B. K.; Cho, M. H. Visible light-induced enhanced photoelectrochemical and photocatalytic studies of gold decorated SnO2 nanostructures. New J. Chem. 2015, 39, 2758−2766. (14) Ni, W.; Liang, F.; Liu, J.; Qu, X.; Zhang, C.; Li, J.; Wang, Q.; Yang, Z. Polymer nanotubes toward gelating organic chemicals. Chem. Commun. 2011, 47, 4727−4729. 3393

DOI: 10.1021/acs.inorgchem.6b02914 Inorg. Chem. 2017, 56, 3386−3393