Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2

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Thermally Stable Hierarchical Nanostructures of Ultrathin MoS2 Nanosheet-Coated CeO2 Hollow Spheres as Catalyst for Ammonia Decomposition Xueyun Gong,†,‡,§ Ying-Qiu Gu,∥ Na Li,† Hongyang Zhao,† Chun-Jiang Jia,*,∥ and Yaping Du*,†,§ †

Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710054, China ‡ College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, China § Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, China ∥ Key Laboratory for Colloid and Interface Chemistry, Key Laboratory of Special Aggregated Materials, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China S Supporting Information *

ABSTRACT: MoS2 ultrathin nanosheet-coated CeO2 hollow sphere (CeO2@ MoS2) hybrid nanostructures with a 3D hierarchical configuration were successfully constructed from a facile two-step wet chemistry strategy: first, CeO2 formed on a silica core which served as a template and was subsequently removed by NaOH solution to attain hollow spheres, and then few-layered ultrathin MoS2 nanosheets were deposited on the CeO2 hollow spheres through a hydrothermal process. As a proof of concept application, the asprepared CeO2@MoS2 hybrid nanostructures were used as catalytic material, which exhibited enhanced catalytic activity in ammonia decomposition for H2 production at high temperature. It was demonstrated that, even with a structural transformation from MoS2 to MoNx under harsh conditions of ammonia decomposition at high temperature (700 °C), the 3D hierarchical nanostructures of the CeO2@MoNx were well kept, indicating the important role of the CeO2 support.

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INTRODUCTION In recent years, two-dimensional (2D) layer structured nanomaterials have attracted increasing interest due to their unique properties and promising applications.1−8 As an important two-dimensional (2D) layer structured nanomaterial, MoS2 has been widely used for diverse applications, such as catalysis,9−14 solid-state lubrication,15 energy storage,16 transistors,17,18 sensing,19−21 and lithium batteries.22−24 As far as the catalysis applications for MoS2 are concerned, both theoretical calculations25 and experimental results26 have indicated that the catalytic activity mainly originated from active sites located along the edges of MoS2 layers. However, the freshly prepared MoS2 layers would easily tend to restacking and aggregation, especially during the high-temperature reaction like ammonia decomposition, because of their high surface energy and interlayer van der Waals attractions, which severely reduced the active sites and damped the performances concerning the practical applications. Therefore, how to avoid the stacking and aggregation, thus increasing the exposed active sites, is of special importance. Practically, rational design and synthesis of three-dimensional (3D) hierarchical architectures based on 2D nanosheets is an effective approach for preventing stacking and aggregation, © XXXX American Chemical Society

thus opening the door of realizing practical applications of this kind of nanostructure.27−29 Ammonia (NH3), which can be easily liquefied under moderate conditions, has been considered as an excellent storage medium for hydrogen compared with traditional carbonaceous materials.30 Catalytic ammonia decomposition is an efficient method to generate COx-free hydrogen. Up to now, Ru has been found to be the best ammonia decomposition catalyst.31 Transition metals based on Fe and Ni have also been investigated as excellent catalysts for NH3 decomposition.32−35 Recently, molybdenum nitrides were found exhibiting high performance as catalyst for catalytic decomposition of ammonia to produce COx-free hydrogen.36 Molybdenum nitrides can be obtained by ammonolysis of MoS2,37 which gave the possibility for MoS2 used for catalytic ammonia decomposition. Ceria (CeO2) has been studied intensively for its unique properties and found to act as a highly effective support or promoter for catalytic ammonia decomposition to hydrogen. 38,39 Up to now, diverse morphologies of CeO 2 nanostructures have been fabricated, such as nanocubes,40 Received: February 1, 2016

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DOI: 10.1021/acs.inorgchem.6b00265 Inorg. Chem. XXXX, XXX, XXX−XXX

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stainless steel autoclave and heated in an electric oven at 130 °C for 24 h. The autoclave was cooled down to room temperature. The SiO2@ CeO2 particles were collected by centrifugation (10 000 rpm) and washed with absolute ethanol. The products were dispersed in 5 mol/ L NaOH solution for 2 days to attain CeO2 hollow spheres for further use. Synthesis of MoS2 Nanosheets. In a typical procedure, 50 mg of sodium molybdate, 100 mg of thioacetamide, and 14 mL of deionized water were added into a 25 mL Teflon-lined stainless steel autoclave. After stirred for 30 min, the suspension solution in Teflon-lined stainless steel autoclave was heated in an electric oven at 200 °C for 24 h. The autoclave was then cooled down to room temperature. The black precipitate was collected by centrifugation, washed thoroughly with absolute ethanol, and dried at 65 °C overnight in vacuum oven. The as-prepared MoS2 nanosheets were further treated at 600 °C in Ar atmosphere for 4 h to obtain high crystallinity. Synthesis of CeO2@MoS2 Hybrid Nanostructures. In a typical procedure, a 20 mg portion of the obtained CeO2 hollow spheres was dispersed in 14 mL of deionized water with ultrasonication for 30 min. Then, 50 mg of sodium molybdate and 100 mg of thioacetamide were added. After being stirred for 30 min, the suspension solution was transferred into a 25 mL Teflon-lined stainless steel autoclave and then heated in an electric oven at 200 °C for 24 h. The autoclave was then cooled down to room temperature. The black precipitates were collected by centrifugation, washed thoroughly with absolute ethanol, and dried at 65 °C overnight in a vacuum oven. The as-prepared CeO2@MoS2 hybrid nanostructures were further treated at 600 °C in Ar atmosphere for 4 h to obtain high crystallinity. Characterization. The morphologies of the as-obtained products were examined by the transmission electron microscopy (TEM, Hitachi HT-7700) and scanning electron microscopy (SEM, FEI Quanta F250). The detailed microscopic structure and the chemical composition of the products were analyzed using high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN) operated at 200 kV. The crystallinity and crystal phase of the dried powders of samples were examined by X-ray diffraction (XRD, Rigaku D/MAX-RB) with a scanning rate of 5°/min from 10° to 80°, using Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption−desorption isotherms were measured on a Micromeritics TriStar 3000 porosimeter (mesoporous characterization) and Micromeritics ASAP 2020 (microporous characterization) at 77 K. The samples were outgassed at 60 °C for 12 h under vacuum prior to measurements. The specific surface areas were calculated on the basis of the Brunauer− Emmett−Teller (BET) method, and the pore size distributions were measured from the desorption branch of isotherms using the Barrett− Joyner−Halenda method (BJH). Energy dispersive X-ray spectrometry (EDX) characterizations were performed on a transmission electron microscope (Tecnai G2 F20 S-TWIN). X-ray photoelectron spectroscopy (XPS) data were obtained using a Escalab 250 xi photoelectron spectrometer using Al K radiation (15 kV, 225 W, base pressure ∼5 × 1010 Torr). In Situ XRD Experiments. In situ XRD experiments under hydrogen atmosphere were carried out on a PANalytical X’Pert3 Powder diffractometer operating in reflection mode with Cu Kα radiation (λ = 1.54178 Å, 40 kv, 40 mA). The powder sample was loaded into a ceramic tube which was attached to an in situ flow cell (Anton Paar HRK-900 reaction cell). A small resistance heating wire was installed right below the tube, and the temperature was monitored with a chromel−alumel thermocouple that was placed inside the tube near the sample. The in situ XRD tests (5% H2/Ar, 30 mL/min) were carried out following a temperature-programmed mode: 25 °C → 100 °C → 200 °C → 300 °C → 400 °C → 500 °C → 600 °C → 700 °C → 800 °C → cool down (ramping rate: 30 °C/min) with stabilization at each temperature for 60 min. Data were collected with a step width of 0.013°, and a counting time of 50 s per step (20 min/run). Data obtained from the last run at each temperature was used for plotting. Catalytic Testing. For a typical ammonia (NH3) decomposition experiment, 100 mg (20−40 mesh) of the catalyst was loaded in a quartz tube (I.D. = 6 mm) fixed bed reactor, and pure gaseous NH3 was passed through the catalyst bed. The reactor temperature was

nanorods, 41,42 nanotubes,43−45 etc. Among the various morphologies, the porous hollow spheres46,47 with remarkable interior space and pore structure are particularly attractive because of their low density, high specific surface area, and superior permeation. Herein, for the first time, we reported the growth of 2D ultrathin MoS2 nanosheets on uniform CeO2 porous hollow spheres to form ultrathin MoS2 nanosheet−CeO2 hollow sphere hybrid nanostructures with a 3D hierarchical configuration, which is denoted as CeO2@MoS2. The CeO2@MoS2 hybrid nanostructures and MoS2 nanosheets were used as catalysts for the production of hydrogen via catalytic decomposition of ammonia. It is found that, even through a structural transformation from MoS2 to MoNx under a harsh condition of ammonia decomposition at high temperature (700 °C), the 3D hierarchical nanostructures of the CeO2@MoNx were kept well (Scheme 1), indicating the important role of the Scheme 1. Schematic Illustration for the Synthesis of CeO2@ MoS2 Hybrid Nanostructures and Its Use in Catalytic Decomposition of Ammonia for H2 Production

CeO2 support, which induced the enhanced catalytic activity. However, the pure MoS2 nanosheets exhibited poor catalytic activity due to the serious aggregation during the catalytic process. Although the synergistic effect between Mo- and Cebased materials is still not clear, the design and synthesis of hybrid hierarchical materials spot a light on the preparation of highly efficient and stable catalysts in heterogeneous catalysis.

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EXPERIMENTAL SECTION

Materials and Reagents. Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99.99%, Sigma-Aldrich), tetraethylorthosilicate (Si(OC2H5)4, 98%, Acros Organics), sodium molybdate (Na2MoO4· 2H2O, 99%, Fluka), thioacetamide (CH3CSNH2, 98%, Sigma-Aldrich), ammonium hydroxide (NH3·H2O, 25%), absolute ethanol (C2H6O, 99.7%), sodium hydroxide (NaOH, 96%), and ethylene glycol (C2H6O2) were purchased from Tianjin Zhiyuan Chemical Company. All chemicals were used without any further purification. Deionized water was used throughout this study. Synthesis of CeO2 Hollow Spheres. The CeO2 hollow spheres were synthesized via a slightly modified method developed by Stucky et al.47 In a typical procedure, a mixture of deionized water (56 mL) and ammonium hydroxide (8.4 mL) was added into a flask containing the mixture of ethanol (280 mL) and tetraethylorthosilcate (8 mL) under vigorous stirring at room temperature for 24 h; the products were then collected by centrifugation (8500 rpm), followed by drying at 60 °C for 6 h. A 100 mg portion of dried SiO2 was dispersed in 13 mL of ethylene glycol with ultrasonication in a beaker. Then, 0.5 g of cerium nitrate hexahydrate and 0.75 mL deionized water were added, and the mixture was stirred for 30 min to form a homogeneous solution. The mixture was then transferred to a 25 mL Teflon-lined B

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Figure 1. (a) XRD patterns of CeO2@MoS2 hybrid nanostructures, CeO2 hollow spheres, and pure ultrathin MoS2 nanosheets. In situ XRD patterns collected under hydrogen atmosphere 5% H2 in Ar of (b) pure MoS2 nanosheets and (c) CeO2@MoS2 hybrid nanostructures.

Figure 2. SEM images of (a) CeO2 hollow spheres and (b) CeO2@MoS2 hybrid nanostructures. TEM images of (c) CeO2 hollow spheres and (d) CeO2@MoS2 hybrid nanostructures and (e, f) HRTEM images. (g) HAADF-STEM image of CeO2@MoS2 hybrid nanostructures. increased from 400 to 700 °C in 50 °C steps. At each step, the reaction was allowed to equilibrate for 60 min to reach the steady-state conditions, and data obtained from the last gas chromatography run at each temperature were used to calculate the conversion value. Reaction temperature was maintained at 700 °C for 58 h to evaluate the stability of the catalyst, and the NH3 conversion was recorded continuously. After the stability test, the temperature was decreased to ambient temperature under NH3 flow and then increased from 400 to 700 °C in 50 °C steps. The NH3 conversion data were collected again during the heating procedure. The concentrations of outlet gases were analyzed by an online gas chromatograph (Ouhua GC 9160), which was equipped with a thermal conductivity detector (TCD) and Porapark Q column (1.5 m of length).

MoS2 nanosheets were examined with powder X-ray diffraction analysis (XRD, Figure 1a). The diffraction peaks of CeO2 hollow spheres can be readily indexed to the cubic phase with lattice constant of a = b = c = 5.41 Å (space group Fm3̅m, JCPDS 34-0394). As for the pure ultrathin MoS2 nanosheets, the crystal phase mainly consists of hexagonal MoS2, but the discernible peaks cannot match the standard MoS2 card (JCPDS 37-1492) exactly. The diffraction peaks corresponding to MoS2 were not observed in the XRD pattern of CeO2@ MoS2 hybrid nanostructures, indicating that MoS2 nanosheets coating on the CeO2 hollow spheres may consist of only few layers, which are too thin to be detected.9,22,48−51 Due to the reducibility of produced H2 in catalytic NH3 decomposition conditions, it is necessary to monitor the crystal phase changes of catalyst under H2 atmosphere at the reaction temperature. The corresponding in situ XRD results were

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RESULTS AND DISCUSSION The crystal phases of as-harvested CeO2@MoS2 hybrid nanostructures, CeO2 hollow spheres, as well as ultrathin C

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Figure 3. EDX elemental mapping images of Ce, O, Mo, and S in the selected area showing in Figure 2g.

eV) and Ce 3d94f1Ln (885.5 eV) states corresponded to vo and v1. The u (u = u, u0, u1, u2, u3) structures corresponding to the Ce 3d3/2 levels could be characterized by the same assignment.53,54 Therefore, the XPS results indicated that Ce in the as-synthesized CeO2@MoS2 hybrid nanostructures was mainly cerium(IV) and cerium(III). The fitting of the O 1s region with two-peak contribution elucidated that at least two kinds of oxygen species were present in the near surface domain of CeO2@MoS2 hybrid nanostructures (Figure 4b). The peak at about 530.0 eV was due to crystal lattice oxygen of CeO2, while the peak at about 532.5 eV was due to chemisorbed oxygen on the hybrid nanostructure surfaces. The binding energies of Mo 3d3/2 and Mo 3d5/2 peaks were located at 231.9 and 228.7 eV, respectively, suggesting that the oxidation state of the Mo ion is positive quadrivalence in the synthesized CeO2@MoS2 hybrid nanostructures (Figure 4c).28 Meanwhile, the peaks at 161.6 and 162.6 eV in Figure 4d could be assigned to the binding energies of S 2p3/2 and S 2p1/2, respectively; thus, the S was negative divalent in the CeO2@MoS2 hybrid nanostructures. Full nitrogen sorption isotherms of as-synthesized CeO2@ MoS 2 hybrid nanostructures were recorded to attain information on the pore size distribution and specific surface area. A type IV nitrogen isotherm shown in Figure 5a suggested a characteristic of mesoporous materials.55 Accordingly, the Brunauer−Emmett−Teller (BET) specific surface area of CeO2@MoS2 hybrid nanostructures was about 54.9 m2 g−1. The pore size distribution derived from adsorption data and calculated by the Barrett−Joyner−Halenda (BJH) method was shown in Figure S2. The plot indicated that most pores were in the mesoporous structure with the pore diameter of approximately ∼4 nm. Such a high specific area of hollow structures with mesoporous structure could be desirable for catalytic applications, which may facilitate the transportation of mass and reactants. The catalytic activity of each sample for the hydrogen production via decomposition of ammonia was tested in two identical runs, in which the first heating run was regarded as the D

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Figure 4. XPS spectra of CeO2@MoS2 hybrid nanostructures: (a) Ce 3d, (b) O 1s, (c) Mo 3d, and (d) S 2p.

obviously higher than that of the former, demonstrating that the transformation of MoS2 to molybdenum nitride in Figure 5b was incomplete and a simple activated process would not fully exhibit the actual catalytic ability of catalysts. It may be the reason that the catalytic activity of CeO2@MoS2 hybrid exhibited lower activity compared with the previously reported MoOx catalyst.56 To investigate the phase changes, structure, and texture of the CeO2@MoS2 hybrid nanostructures, characterizations on the reacted catalysts after the NH3 decomposition have been carried out. As seen from Figure 6a, the crystal phases of the used CeO2@MoS2 hybrid nanostructures and pure ultrathin MoS2 nanosheets after a stability test were examined with XRD. Ammonolysis of MoS2 at 700 °C for 58 h led to a mixture of the molybdenum nitride phase, and the XRD diffraction patterns were assigned to hexagonal MoN (JCPDS 25-1367) and hexagonal Mo5N6 (JCPDS 51-1326), respectively. There still exists unreacted molybdenum sulfide phase which corresponded to monoclinic Mo2S3 (JCPDS 40-972). For CeO2@MoS2 hybrid nanostructures, molybdenum sulfide transformed to molybdenum nitride completely after reaction at 700 °C for 58 h, and CeO2 phase was transformed to hexagonal Ce2O2S (JCPDS 04-626). For Mo-based catalysts, Tagliazucca et al. have performed in situ XRD to study the phase transformations of MoO3 catalyst during the NH3 decomposition reaction.56 The phase transformations for molybdenum oxide during the NH3 decomposition process was very complex because nitridation of molybdenum and

self-activation procedure. The catalytic tests were performed over 100 mg of CeO2@MoS2 samples with ammonia (NH3) space velocity of 12 000 cm3 gcat−1 h−1. After activation, the catalytic performance of CeO2@MoS2 hybrid nanostructures for NH3 decomposition reaction, tested as a function of temperature, was presented in Figure 5b. As can be seen, the activity in terms of NH3 conversion increased with elevating the reaction temperature. The content of MoS2 in CeO2@MoS2 hybrid nanostructures was ca. wt 62%; however, the NH3 conversion of CeO 2@MoS2 hybrid nanostructures was obviously higher than that of pure MoS2 nanosheets when the same amount of catalysts was adopted (100 mg), indicating CeO2 serves as an excellent promoter which could effectively improve the catalytic activity of MoS2. To further demonstrate the high-temperature stability of catalysts, long-duration stability tests were performed for pure MoS2 nanosheets and CeO2@MoS2 hybrid nanostructures at 700 °C with a gas hourly space velocity (GHSV) of 12 000 cm3 gcat−1 h−1 over 58 h. As depicted in Figure 5c, the catalytic activity of CeO2@ MoS2 hybrid nanostructures was much higher than that of pure MoS2 nanosheets during the whole process. The conversion of two samples remained increased over the initial 34 h and subsequently stabilized at nearly 100% conversion, indicating that nitridation of MoS2 to molybdenum nitride was a very slow process. Figure 5d showed the comparison of catalytic activity for CeO2@MoS2 hybrid nanostructures between activation by a first heating procedure (Figure 5b) and by the stability test at 700 °C for 58 h. The NH3 conversion of the latter was E

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Figure 5. (a) N2 adsorption/desorption isotherms of CeO2@MoS2 hybrid nanostructures. (b) Temperature dependent NH3 conversion at a GHSV of 12 000 cm3 gcat−1 h−1 after activation by a first heating process. (c) Long-term catalytic stability of the catalysts measured at 700 °C at a GHSV of 12 000 cm3 gcat−1 h−1 (100 mg, 20 mL/min). (d) Comparison of catalytic activity for CeO2@MoS2 hybrid nanostructures between activation by a first heating procedure and by a stability test at 700 °C for 58 h.

from aggregating.38 In the CeO2@MoS2 hybrid nanostructures, we believe that the CeO2 spheres inside can also enhance the dispersion of the active molybdenum sites and suppress MoS2 nanosheets from restacking and aggregation under harsh reaction conditions. Also, the possible electron transfer on the Ce−Mo interface could be accelerated, through which the performance of the CeO2@MoS2 hybrid nanostructures for catalytic decomposition of ammonia was promoted. In addition, XPS was applied to investigate the surface state of the corresponding catalyst after the stability test of Ce, O, S, Mo, and N in CeO2@MoS2 hybrid nanostructures (Figure S3a−e) and pure MoS2 nanosheets (Figure S4a−c). As shown in Figure S3a, only v (v = vo, v1) and u (u= uo, u1) for characteristics of Ce(III), which could be observed in the Ce 3d XPS spectrum, indicate that the oxidation state of Ce(IV) was reduced to Ce(III).54 Meanwhile, XRD (Figure 6a) indicated that CeO2 (Ce(IV)) was reduced to Ce2O2S (Ce(III)) by ammonolysis. The fitting of the O 1s region with the peak at about 530.2 eV is due to the crystal lattice oxygen of Ce2O2S (Figure S3b). Meanwhile, the discernible peak at 161.5 eV in Figure S3c could be assigned to the binding energies of S 2p3/2 of Ce2O2S. Figure S3d showed the peak at 229.8 eV (Mo4+ 3d5/2), 232.28 eV (Mo6+ 3d5/2), and 235.38 eV (Mo6+ 3d3/2), suggesting the mixed valence state of the Mo species.57 The peaks at 394.4 and 397.9 eV can be assigned to Mo 3p3/2 and N 1s, respectively, which were characteristic of molybdenum nitride (Figure S3e),58 indicating that the molybdenum sulfide

reduction of molybdenum oxide occurred simultaneously. They concluded that many factors, including the active phase, domain size, and surface species/area, could influence the activity of the catalyst. It would be too simple to correlate the catalytic activity only to the phase composition. For the CeO2@ MoS2 hybrid, the phase transformation was more complicated than that for MoO3, so it is impossible to distinguish the contribution of each component. The morphology of CeO2 (Ce2O2S) hollow spheres had no substantial changes due to its excellent thermal stability (Figure 6b). However, pure MoS2 (MoN x ) nanosheets seemed to severe restacking and aggregation after the harsh reaction conditions (temperature up to 700 °C, large concentration of ammonia and/or hydrogen) as can be observed from the TEM and HAADFSTEM (inset) image in Figure 6c. For CeO2@MoS2 (Ce2O2S@ MoNx) hybrid nanostructures (Figure 6d), the CeO2 (Ce2O2S) hollow structure was well-maintained, and no major agglomeration occurred in the MoS2 (MoNx) nanosheets coated on the surfaces of CeO2 (Ce2O2S) spheres. Noticeably, even after reaction at 700 °C for 58 h, the morphology of CeO2@MoS2 (Ce2O2S@MoNx) hybrids were still well-maintained (STEM image in inset of Figure 6d). Meanwhile, the specific surface area of the used CeO2@MoS2 (Ce2O2S@MoNx) hybrid nanostructures calculated from nitrogen sorption isotherms was 37.5 m2g−1. It has been reported that CeO2 acts as a dispersant in the Ni−Al catalyst system and supplies “spacers” to inhibit the mobility of Ni atoms, preventing the particles F

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Figure 6. TEM images of the reacted (a) XRD patterns of the reacted CeO2@MoS2 (Ce2O2S@MoNx) hybrid nanostructures and pure MoS2 nanosheets after stability test. (b) CeO2 (Ce2O2S) hollow spheres, (c) MoS2 (MoNx) nanosheets, and (d) CeO2@MoS2 (Ce2O2S@MoNx) hybrid nanostructures after two heating runs (insets in c and d represent HAADF-STEM images of corresponding catalyst after stability test).

transformed to molybdenum nitride after reaction at 700 °C for 58 h. Figure S4a−c showed the XPS signals taken from the Mo 3d, S 2p, Mo 3p, and N 1s regions of the corresponding catalyst after the stability test of pure MoS2 nanosheets. As seen from Figure S4a, XPS signals at 235.6 and 232.5 eV corresponded to Mo6+ 3d3/2 and Mo6+ 3d5/2, respectively. The peaks at 228.8 eV in Figure S4a could be assigned to the binding energies of Mo4+ 3d5/2 of MoS2. Double peaks at 162.8 and 161.7 eV (Figure S4b), attributable to the core levels of S 3p1/2 and S 3p3/2, respectively, were characteristic of S2− in MoS2. While the peaks located at 394.3 and 397.8 eV were ascribed to the levels of Mo 3p3/2 and N 1S of molybdenum nitride (Figure S4c), it is noticed that the XRD results showed the samples after the catalytic tests were mainly composed of Mo2S3 and MoNx, indicating that Mo should be reduced from +4 to even lower oxidation state in the reductive reaction condition. However, Mo6+ was observed from the XPS spectra, which is perhaps because the surface of the sample was oxidized when it was exposed to air since XPS is a surface-sensitive technique.

surface area and exhibited enhanced activity in catalytic ammonia decomposition for H2 production. The preparation of CeO2@MoS2 hybrid nanostructures and use of it as catalyst for ammonia decomposition may pave the way toward facile and efficient production of ultrathin transition metal dichalcogenides and rare earth materials-based catalysts with excellent performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00265. Figures S1−S4 (TEM, pore size distribution, and XPS data) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

CONCLUSION In summary, for the first time, we reported the synthesis of 2D ultrathin MoS2 nanosheets grown on uniform CeO2 hollow spheres to form CeO2 hollow spheres@MoS2 nanosheets hybrid nanostructures with a 3D hierarchical configuration. The as-prepared CeO2@MoS2 hybrid nanostructures possessed high

Author Contributions

X.G., Y.-Q.-G., and N.L. contributed equally to this work. Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS We gratefully acknowledge the financial aid from the start-up funding from Xi’an Jiaotong University, the Fundamental Research Funds for the Central Universities (2015qngz12), Fundamental Research Funding of Shandong University (Grant 2014JC005), the Taishan Scholar project of Shandong Province (China), the NSFC (Grants 21371140, 21301107), the National Science Funds of China for Excellent Young Scientists (Grant 21522106), and the Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (Grant 338080051). We also appreciated Dr. Xinghua Li at Northwest University for his kind help to obtain HRTEM images.

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DOI: 10.1021/acs.inorgchem.6b00265 Inorg. Chem. XXXX, XXX, XXX−XXX